Same Species in Other Localities.

There are three specimens in the museum collection from East Bethany, N. Y., which are labelled C. mucronatus but whose sharp, distant striæ and smooth umbos show that they belong to the species here described. One is 6 x 8 mm. and has 20 striæ; a second is 6 × 9 mm. with 24 striæ and the largest is 8 X 10 mm. and bears 22 striæ and 4 pairs of spines. They are slightly less gibbous than the Canandaigua Lake specimens but are in other respects the same.

On the beak and umbo of the ventral valve of one specimen are growth lines marking the early stages. The smallest of these is .35 mm. long and .41 mm. wide and the next is .65 mm. long and .91 mm. wide. There is a narrow sinus which does not extend quite to the second growth line but that line probably marks the limit of the nepionic shell. It is slightly convex, and the hinge width is about equal to the width below.

The plications begin beyond a line 2.2 mm. from the beak. At this stage the shell is 3.6 mm. wide, and the cardinal extremities are a little extended.

Chonetes: Summary.

All the species of Chonetes here described have, as would be expected, the same general type of changes in development. The different species are here compared at the various stages.

Protegulum. The shell in this stage is exceedingly small, but in none of the specimens in the present material are the beaks well enough preserved to show it. From Beecher's work we know that in C. scitulus the protegulum is nearly circular in outline (.117 mm. long and .III mm. wide, according to the figure) with a strongly arcuate hinge.

Nepionic Stage. The species agree in having the shell at this stage convex in the ventral valve and convex at the umbo and concave or flat in front in the dorsal valve. This is an advance on the condition in Stropheodonta where the dorsal shell was convex during the whole of the nepionic stage.

The ventral valve always has a narrow sinus and the dorsal valve a corresponding median fold and usually two less definitely marked lateral folds.

The outline is subcircular, though the width is usually a little greater than the length. The hinge is somewhat arcuate. The length of the

nepionic shell varies considerably in the different species, but is always less than 1 mm. It is least in C. coronatus and greatest in C.

mucronatus.

Pauciplicate Neanic Stage. In this stage new striæ are introduced by implantation and not by the bifurcation of the older striæ. The striæ are strong, simple, and separated by spaces as wide as the striæ. The number of the striæ is not great. In C. coronatus and C. scitulus the largest number is usually 13 on the ventral valve and 12 on the dorsal. In the other species there are more, but usually less than 20.

The shell is about one fifth wider than long, and gently concavoconvex. The first spines (usually two or three pairs) appear during this stage. The size of the shell at the end of this stage, that is, when the striæ begin to bifurcate, is least in C. scitulus, where it is about 2 mm. in length, and greatest in C. robustus, where it reaches a length of 6.5 mm.

Later Neanic and Ephebic Stages. It would be difficult to make any sharp line which would mark the end of the adolescent period and the assumption of all the adult characters. The size, convexity of valves and number of striæ continue to increase during these stages; the width becomes greater in proportion to the length and a number of pairs of spines are added on the cardinal margin.

Gerontic Stage. Senile characters are not well shown except in C. mucronatus. In all, this stage seems to be accompanied by a thickening of the shell, a deepening of the muscle scars and a growth. of the anterior part of the shell, which reproduces the early neanic condition of length almost equal to breadth; and an increase in the convexity of the ventral valve.

The simplest type of Chonetes would then seem to be one with a moderately convex ventral valve, the width a little greater than the length, the pedicle tube open at maturity, the surface covered with simple. striæ and with few pairs of spines. Of the species here discussed C. robustus is the most primitive in its character, as is shown by the continuation of the smooth state into the early neanic stage, the large size of the shell and the comparatively large number of striæ acquired before the end of the "pauciplicate" neanic stage, and the retention of the open pedicle tube in the adult.

Of the species in the older formations, C. cornuta Hall of the Clinton and C. undulata Hall of the Niagara show some of the primitive

characters. Hall, in describing the former species in Vol. II, Pal. N. Y., p. 64, says: 'Surfaces covered with fine equal striæ which are equal to the spaces between them: striæ round, straight, bifurcating. Three spines on each side of the beak. The species is more flat than the small one in the Corniferous and Hamilton group which it resembles." In the 11th Indiana Report, p. 293, Hall says of the same species: ". . . in C. cornuta the interstices are wider than the striæ, and the latter increase only by interstitial additions below the middle of the shell." The size of the shell, according to Hall's figures, is about 4.2 X 5.5 mm.

In his descrip

Chonetes undulata, in the specimens figured,' has 21 simple striæ on the ventral valve and is 3 mm. long by 4.75 mm. wide. tion Hall says "Surface marked by obscure radiating plications which are broad and rounded below and obsolescent toward the beak: cardinal margin of the ventral valve ornamented by two diverging spines on each side of the beak. . . . The bases of two spines on each side of the beak are usually preserved and no specimen examined has more than that number."

Stropholasia truncata Hall.

(Plate VII., Rows 3 and 4.)

Hall, Pal. N. Y., Vol. IV, 1867, p. 160, pl. 23, figs. 12-24.

Of this species there are about 50 good specimens in the collection. There are about as many of one valve as of the other. The largest specimen is 8.33 mm. long and 10 mm. wide: The smallest 2.06 X 2.69 mm. All of the specimens from this material are small for the species. The smallest specimen figured by Hall is 10 mm. long and 12 mm. wide while the largest is 17 mm. long and 18 mm. wide.

FIG. 27.

Stropholosia

truncata Hall; dorsal Nepionic Shell.- Owing to the deformation of the venbeak, show- tral beak and umbo resulting from the method of attaching protegu- ment, very little could be made out concerning that valve in the youngest stages except that it is regularly and moderately convex. The smallest ventral valve is 2.93 mm.

lum and nepionic shell. X 16.

long, and 3.33 mm. wide. The beaks of some of the dorsal valves are extremely well preserved and all stages from the protegulum to the adult can readily be observed. The protegulum is transversely oval, with a gently curved hinge. In the best preserved specimen it is .131 mm. long and .156 mm. wide (Fig. 27). 1 Eleventh Ann. Report, Indiana State Geol., p. 294, pl. 22, fig. 15.

The dorsal valve of the nepionic shell is subcircular in outline, with the hinge line equal to, or greater than, the width below. It is convex on the umbo and sometimes for its whole length, but is generally concave in front. The surface is smooth, without spines.

Outline.

Changes During Development.

The general outline remains much the same throughout life. In the later stages the width is slightly greater, in proportion to the length, than in the younger specimens. The index varies from 1.10 to 1.25. There are a few individuals with higher indices, one of 1.27 and one of 1.35. The length of the hinge is always less than the greatest width in all stages after the nepionic.

Convexity of Valves. In the neanic stages the ventral valve is moderately convex and the dorsal valve convex at the umbo and concave in front. In the adult, the ventral valve is very strongly convex, while the dorsal valve is correspondingly concave toward the front.

Surface Markings.-There are no striæ at any stage. After the nepionic stage, spines are developed on both valves, but more numerously on the ventral than on the other valve. On the dorsal valve they are generally broken off close to the base, but one specimen retains two of the spines, which are long, slender, and lie against the surface of the valve. On the ventral valve they are better preserved, in some cases most perfectly. They are most abundant along the cardinal margin and stand erect, curving in toward each other from opposite sides of the beak, suggesting that they might have been of use in anchoring the shell. Over the rest of the surface they are

directed forward.

There is one dorsal valve, 3.66 mm. by 5 mm., which is devoid of any trace of spines. This may represent another species.

On all the dorsal valves there are, in addition to the spine bases, four or five pairs of wrinkles near the cardinal margin. Between them are pits which may represent the points of attachment of spines as wherever the spines of the dorsal valve have been detached shallow pits are left in the shell.

from .5

Cardinal Area. The area of the ventral valve is narrow, to .75 mm. in width, and increases only a little after the shell has reached a length of 5 or 6 mm. The delthyrium is completely closed by a convex deltidium. Strong teeth bound the edges of the delthyrium. The dorsal area is about one third the width of that on the

ventral valve. There is a convex chilidium, at the apex of which is the minute anal opening. The inner opening of the anal tube is at the anterior base of the cardinal process, just in front of the point where it bifurcates. The cardinal process in the young stages is wider than long, projects little beyond the hinge line, and is divided once. In the adult it is longer than wide, deeply bifurcated in front and quadrifed on the posterior face. On each side of it is a dental socket. which is bounded posteriorly by a slightly developed lamella. Muscle Scars. - In the dorsal valve there are two pairs of small adductor scars, separated by a thin median septum which extends half way to the front of the valve.

In the ventral valve there are two small adductor scars and in front

FIG. 28. Rhipidomella vanuxemi Hall: A ventral valve show

ing the comparatively large number of striæ at a very early stage, and the smallness of the nepionic shell. X 16.

FIG. 29. The same species. Outline showing relative convexity of valves of a shell .58 mm. in length. Pedicel valve above. X16.

The largest shell

and 32 mm. wide.

of them two larger diductor impressions. scars are not strongly marked in either valve.

Rhipidomella vanuxemi Hall.

The

Hall, Pal. N. Y., 1867, pp. 40, 47, pl. 5, fig. 6, pl. 6, fig. 3.

There are about 150 shells belonging to this species in the collection. Nearly all the small shells and many of the larger ones retain both valves in their proper relations. The large ones are not well preserved and the largest one which retains its form with sufficient completeness to be measured, is only 15 mm. in length, while there are fragments of larger ones, some indicating a shell of more than twice that size. The larger

shells are, however, rare. The smallest shell is .42 mm. in length and .61 mm. wide.

of this species, figured by Hall, is 27 mm. long The smallest, 13.5 × 15 mm.

Nepionic Stage. The non-plicate shell in this species is extremely small, varying from .10 to .25 mm. in length.

Both valves are convex, without fold or sinus. The outline is subcircular, the length and width nearly equal, and the hinge arcuate (Figs. 28, 29).

Outline.

Changes During Development.

In the early neanic stages the shell is wider than long and the frontal and posterior margins are nearly parallel, which gives the shell a pronounced transverse appearance. The index is from