Tkopidoleptus Fauna at Canandaicua Lake, N. \'. 145 



tended as some of the Eighteen Mile Creek specimens. In the earlier 

 stages the shells were more mucronate. The neanic stages of all three 

 varieties seem to have been about the same, as all show the short 

 shell, the extended ca-rdinal extremities and the depressed line in the 

 dorsal fold. In the first variety, the later neanic and adult growth 

 has been transverse, while in the other two forms it has been longi- 

 tudinal as well. All these varieties contain specimens which present 

 senile conditions. Out of 680 specimens examined 20 per cent. ( 136 

 specimens) showed senile characters. These characters are most com- 

 monly expressed at the Thedford locality by strong varices of growth 

 near the anterior margin of the shell which increase the depth of the 

 valves and produce a broad flat anterior face in place of the sharp 

 junction of the valves in the normal shell. The shells also become 

 thickened, and lateral growth stops. 



Of 400 adult specimens, 182 showed a trace of the depressed line in 

 the fold of the dorsal valve, while the remainder, 55 per cent., did 

 not. Of those which showed the depressed line, many had only the 

 faintest trace of it, while others, 7 per cent, of the whole, had a well 

 marked sinus along the median line of the fold. To this class belong 

 all of variety one, the short wide form, and a few of the third variety. 

 None of the specimens show the plication in the sinus of the ventral 



valve. 



Delthyris consobrinus d'Orbigny. 



Hall, Pal. N. Y., IV, 1S67, pi. 35, figs. 15-33. 



There are about 50 specimens in the collection belonging to this 

 species. They are all small for the species. The largest is 9.4 mm. 

 long and 17.4 mm. wide, and the smallest is 1.28 mm. by 1.38 mm. 

 The specimen figured by Hall as an individual of average size is 15 

 mm. long and 25 mm. wide, while the largest figured by him is 17 x 

 32 mm. and the smallest 6 X ic.6 mm. 



The developmental changes are about the same as in Spirifcr mucro- 

 nattis, the only striking difference being that the valves are subecpially 

 con\ex in the early neanic stages and the ventral valve becomes much 

 deeper than the dorsal in the mature shell, while in 5. inucronatus the 

 reverse is the case. 



Nepioiiic Stai:;c. — In the nepionic stage the shell is biconvex and 

 oval in outline. It is broader than long, the hinge width less than 

 the width below. The surface is smooth. The length is from .35 to 

 .50 mm. In one specimen the dimensions are .41 x -53 nun- 



