Tropidoleptus Fauna at Canandaku'a I^akk, N. V. 113 



There is no great variation from this outline during the life history, 

 although some individuals have unusual proportions. The average 

 index, for adult specimens, is about 1.30. 



Convexity of Valves. — In the nepionic stage the ventral valve is 

 slightly convex. The dorsal valve is convex for about half its length 

 and then becomes concave. The ventral valve is only moderately 

 convex in the earlier neanic stages, but in later neanic and ephebic 

 stages it becomes strongly so. The dorsal valve follows the contour 

 of the opposite one quite closely. 



Strice. — The smallest shell, a ventral valve, shows six stri^. Each 

 of the ridges which bounds the median sinus divides, making four 

 strise, and one is added on each margin. In the next stage a striation 

 is implanted in the sinus and two more are added on the lateral mar- 

 gins, making nine in all. The smallest dorsal valve in the collection, 

 I mm. long and 1.12 mm. wide, shows a condition corresponding to 

 this stage. It has eight stri;^, the middle pair of which originated by 



Fig. 25. Chonetes sciltibis Hall ; ventral and dorsal valves, showing fold and 

 sinus and plications of very young specimens. X ^ 2. 



the bifurcation of the median fold of the nepionic shell. In the next 

 stage, four more striiie are added, on each valve, outside the older 

 ones, making 12 on the dorsal and 13 on the ventral valve. From 

 that i)oint new striae are added in front by implantation and bifur- 

 cation. The whole process is the same as in C. coroiiaius. The 

 adult shell has from 30 to 40 stri^, counted on the anterior margin. 



Hinge Development. — The area is narrow on both valves. That 

 of the dorsal valve is a little more than half as wide as the ventral 

 area. In one adult specimen, the ventral area was .44 mm. wide at 

 the apex and the dorsal area, .25 mm. 



The delthyrium is wide, the upper third covered by a convex del- 

 tidium and the remainder filled by the cardinal process. The deltid- 

 ium continues to grow throughout life but is so curved back in its 

 lower part that the cardinal process fills the larger part of the opening. 

 No pedicle opening has been observed on the specimens in this col- 

 lection. On the dorsal valve there is a very narrow chilidium at the 

 apex, covering the base of the cardinal process. In nearly all speci- 



