Tropidoleptls Fauna at Canandaigua Lakh, N. \'. 115 

 Chonetes mucronatus Hall. (Plate V, Rows 4 and 5.) 



Hall, Pal. N. v., I\', 1S67, pi. 20, hg. I, pi. 21, fig. I. 



Of this species there are about 1500 specimens in the collection, 

 nearly all of them detached valves. All stages of growth are shown. 

 The smallest specimen is 1.09 mm. long and 1.13 mm. wide; the 

 largest, 11 mm. long and 13 mm. wide. Hall does not illustrate any 

 specimens of this species from the Hamilton. Those that he figures 

 from the Corniferous and Marcellus are smaller and less convex than 

 the major part of the adults in this collection. The smallest speci- 

 men figured by him is 5.6 x 6.8 mm. and the largest 6.4 X 8.2 mm. 



Nepioiiic Shell. — In the nepionic stage the shell is subcircular in 

 outline, the ventral valve convex, and the dorsal valve convex at the 

 umbo and concave in front. There is a faint median sinus in the 

 ventral valve, but the ridges bounding it are not as strong as in the 

 other species. On the dorsal valve there is a sharp median fold which 

 extends only about half way to the front and then dies out. Faint 

 traces of a pair of lateral folds can be seen. The size of the shell at 

 this stage varies in the different individuals. For one specimen the 

 dimensions are: length .59 mm., width .62 mm., width of the hinge 

 .56 mm., length of the dorsal fold .37 mm. In other specimens the 

 nepionic shell reaches a length of .75 to .90 mm. 



CJiaiiges During Devclopinciit. 



Outline. — At the beginning of the neanic stages the hinge length 

 becomes as great, and then greater than the width below and remains 

 greater until senile characters begin to appear. In all stages the 

 width of the shell is somewhat greater than the length. The greatest 

 excess of width over length occurs in the later neanic and early ephebic 

 stages when the index is about 1.30 to 1.40 mm. In the earlier neanic 

 stages the index is i.io to 1.20 mm. and with the accession of the 

 gerontic characters it falls to 1.25 mm. or even less. 



Convexity of Valves. — In the neanic stages the \entral valve is very 

 moderately convex and the dorsal valve slightly concave, often nearly 

 flat. In the adult and gerontic stages the ventral valve becomes 

 strongly convex and finally gibbous. The dorsal valve is rather 

 strongly concave a little anterior to the middle. 



StricB. — Owing to the low, rounded, often indistinct character of 

 the striae, it was not possible to make out clearly all the steps by which 



