110 Annals ok ihk Carnk(;ik Muskum. 



3.5 nun. long and 4 to 4.5 mm. wide. Then the striae begin to in- 

 crease by both bifurcation and implantation. During the latter neanic 

 stages there are from 30 to 60 striae and, in the adult, 80 to 100. 



The implantation of new striae seems to be due to a mechanical 

 cause, namely, the thinness of the shell. When the striae on one 

 valve bifurcate, a depression is, of course, formed between them. As 

 the other shell conforms to this irregularity on the frontal margin, a 

 single striation is produced. The increase of striae by implantation 

 does not seem to be confined to either valve, but occurs on both. 

 One specimen, in which bifurcation of the striae has just begun, shows 

 this process well. On the dorsal valve there are twelve primary stride, 

 one of which, near the middle of the valve, has bifurcated. On the 

 ventral valve there are thirteen striae, two of which have bifurcated. 

 On the dorsal valve, there are two implanted strice which correspond 

 in position to the depression between the bifurcated striae on the 

 opposite valve, while on the ventral valve there is one implanted stria- 

 tion corresponding to the bifurcated one on the dorsal valve. There 

 is, however, nothing regular in the number of implanted and bifur- 

 cated striae on each valve, for, as Hall has remarked, sometimes or.e 

 valve will have almost all of its secondary stride added by implantation 

 and again by bifurcation, or by both ways in all proportions. 



N//ii:;e Development. — In the early stages the area of neither valve 

 is well developed and the shell is so fragile that no traces of the del- 

 tidium or chilidium are preserved in any of the small specimens. In 

 the latter neanic and adult specimens there is a narrow cardinal 

 area on each valve. That of the dorsal valve is about one half 

 to three fourths as wide as the ventral area. Covering the apical 

 portions of the delthyrium there is a convex deltidium, while the 

 larger part is filled by the cardinal process. No pedicle opening was 

 observed. 



On the dorsal valve there is a narrow chilidium covering the pos- 

 terior end of the cardinal process and the ends of the dental sockets. 

 The cardinal process is strong, roughly triangular, deeply bifurcated, 

 and each lobe is again divided by a shallow groove. 



In the ventral valve there are two strong striated teeth rising in 

 front of, and below, the cardinal area. They curve upward and in- 

 ward. In the dorsal valve there are deep grooves on each side of the 

 cardinal process which are bounded cui the inside by slight projections 

 from the process and on the outside by strong diverging lamellae. 

 The articulation appears to be the same as in Tropidoleptus. 



