Tropidoi.eptus Fauna at Canandaigua Lake, N. ^'. 143 



the depressed line in the dorsal fold, but only 25, 40 ])er cent., have 

 a plication or raised line in the bottom of the sinus. Four specimens 

 show senile characters. 



From Eighteen Mile Creek there are three well-marked varieties. 

 One, the most common, is rather long, nearly semicircular in form, 

 and has very much prolonged cardinal extremities, and the front regu- 

 larly curved. The third variety is long and comparatively narrow, 

 with rounded cardinal extremities. This form is rather rare and is 

 produced by the stopping of growth along the hinge at the end of the 

 neanic or in early ephebic stages. In the neanic stages the shell is 

 acuminate at the cardinal extremities. 



The index of shells of the first form, when they are so preserved as 

 to retain their cardinal extremities, is very high. One specimen, 1 1 

 mm. long, has an index of 4.45. ^^'ith the spiniform extremities tiie 

 width is 49 mm. Without them, it is 20 mm. As most of the speci- 

 mens had at least a part of each cardinal extremity broken off, the 

 index in this variety varies from 1.80 to 3.00 in ordinary specimens. 

 The plications number from 16 to 44, those shells with the highest 

 index having the greatest number of plications. 



The second variety includes shells with an index of 2.00 to 3.00, 

 7 to 13 mm. long and 13 to 39 mm. wide. They carry from 16 to 

 32 plications. This is the Canandaigua Lake type, but the specimens 

 are rather under normal size. 



The index in the third variety is from 1.40 to 1.94 and the number 

 of plications from 18 to 30. Only six specimens of this variety were 

 contained in a collection of about 300 shells. Four of the six bore 

 22 plications each. The largest or these shells is 20.4 by 31 mm., 

 and the smallest 15 by 21 mm. From the growth varices preserved 

 on the shell it can be seen that these shells (in their neanic stages) 

 passed through forms like varieties one and two. 



Of the 164 specimens of all three varieties examined, all but six 

 had a depressed line in the fold of the dorsal valve, while only 30, 

 18 per cent, had the plication in the sinus of the ventral valve. 

 Only six specimens were found which showed strong senile character- 

 istics. 



According to Grabau, .S'. 7niicronatiis is common below the Encrinal 

 limestone, but rare above it in the Eighteen Mile Creek section. He 

 mentions a specimen, probably of the first variety, which was 15 mm. 

 long and 100 mm. wide. 

 10 



