Tropidoleptus Fauna at Canandaigia 1-akk, N. \'. 10.") 



slightly greater proportion of the area and their acute angles are only 

 .75 mm. from the cardinal extremities. About the same proi)ortion 

 is maintained through all the succeeding stages. A specimen 20 mm. 

 wide on the hinge line has the acute angles located 3 mm. from the 

 cardinal extremities. 



The area of the dorsal valve is linear and its growth is arrested very 

 early in the development. The deltidium of the youngest specimen 

 which retains it has already been described. Specimens under three 

 or four mm. in width usually show the .strong, convex deltidium pro- 

 longed below the beak and within the margin of the nepionic shell, 

 to form a tube for the passage of the pedicle. In many cases the up- 

 per part of this tube is broken away near the surface of the shell and 

 what remains of it appears as a rather stout conical tube with a large 

 opening (Figs. 18, 19, 20). 



In a few specimens the entire tube is retained. It then rises about 

 2 mm. above the beak, tapers gradually, and is perforated by a minute 

 pore at the apex. In older specimens, 5 or 6 mm. in width, the scar 

 of the base of this tube can be seen on the ventral beak and inside the 

 circle formed by it there is a minute pedicle opening. In later stages 

 the opening is hard to detect and seems to be closed entirely in adult 

 specimens. 



In no stage is there much open space at the lower end of the del- 

 thyrium. As the cardinal process increases in size it projects more 

 and more beyond the dorsal beak and the deltidium becomes corre- 

 spondingly convex, so that the two fill the delthyrium completely. 

 The deltidium is, in some cases, grooved in such a way as to conform 

 to the shape of the posterior face of the cardinal process. 



The cardinal process is large, bilobed as seen from the front, and 

 quadrifed on the posterior face. It is supported by the lamellre of 

 the dental sockets which rise rather abruptly as thin curved plates 

 and run transversely under the umbo, curving backward to join the 

 cardinal process. There is little change in the form of the process 

 during life, except that it curves more backward and upward in the 

 later stages until it comes up close to the beak. 



In the ventral valve the delthyrium is limited by two strong dental 

 lamellaj which extend slightly beyond the hinge line. In young and 

 adult stages they are unsupported, but in gerontic shells they are 

 braced by two outward curving plates which connect them with the 

 inside of the shell at the umbo. 



