Tropidoleptus F'auna at Canandaigua Lake, N. \'. I.!7 



of nearly etjual convexity. The ventral area is curved and inclined 

 backward so that the ventral beak projects beyond the dorsal. In 

 stages but little later, when the shell is slightly over i mm. in length, 

 the ventral valve is four or five times as deep as the dorsal. At this 

 time the ventral area is high, and curved, and the beak is a little 

 anterior to the hinge line. In more mature shells the area on the 

 ventral valve may become flat and inclined forward or may be curved 

 and inclined backward, as in the nepionic stage. In some individuals 

 this curvature and backward tilting of the area become very strong. 

 It seems to be a senile character as specimens in which it is very i)ro- 

 nounced show several strong growth lines. 



The height of the cardinal area is variable, but in proportion to 

 the width at the hinge, it it decidedly less in very young stages than 

 in older ones. In two specimens less than i mm. on the hinge the 

 height was .26 of the hinge width. In a specimen 1.02 x i-3 nim. 

 this index was .36; in one 3-33X5-I3 mm., it was .58; and in- 

 42 43 44 45 



Figs. 42-44. Cyrdna hatnillonenyis Hall. Anterior, cardinal and dorsal views 

 of a specimen in the stage before the inception of plications. X 16. 



Fig. 45. The same species. Section through the ventral valve of an adult. 

 showing medium septum and the peculiar double tube at the bottom of the groove 

 formed by the dental lamella'. X ^• 



creased to .69 in an individual 4.43x6.86. This rate of increase 

 does not hold for all specimens, however, and the curvature of the 

 area prevents the accurate determination of this index on the majority 

 of cases. In the largest specimen in the collection which has a flat 

 area, the index is .55. The specimen is 7 x 12.4 mm. and the area 

 is 6.8 mm. high. 



Plications. — When the shell has reached a length of from .45 to 

 .60 mm. a sinus is formed in the ventral valve and very soon after its 

 initiation a fold is produced in the opposite shell. This stage, in 

 which there is no other ornamentation than the fold and sinus, con- 

 tinues for some time. The largest specimen showing this state is 1.5 

 mm. long and 2.26 mm. wide. Shells at this period are almost glob- 

 ular and are difficult to tell from the young of Anibocxlia umbouata, 

 unless carefully examined. 



