Trcipidolkptus Fauna at Canandaigua T.akk, N. \. \'2'-\ 



'The dorsal valve of the nepionic shell is subcircular in outline, with 

 the hinge line ecjual to, or greater than, the width below. It is con- 

 vex on the umbo and sometimes for its whole length, but is generally 

 concave in front. The surface is smooth, without spines. 



C/ia/ii:;es During Dcvelopnieiif. 



Outline. — 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 

 I. ID 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 con- 

 cave 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 striae at any stage. After the 

 ne[)ionic stage, spines are developed on both valves, but more numer- 

 ously 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. 



Cardinal Area. — The area of the ventral valve is narrow, from .5 

 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 delthy- 

 rium. The dorsal area is about one third the width of that on the 



