92 A.WALs OF JHE Carnegie Museum. 



largest 38 X 49 miu. I'rom the Chemung the smallest specimen fig- 

 ured measures ^^ x 39 nini., and the largest 42 x 61 mm. 



Protegiihim. — The protegulum is nearly circular, both valves convex, 

 and with arcuate hinge. It measures .10 x .10 mm. in one specimen 

 and .12 X .12 mm. in another. 



Fig. 10. S/ ropkeodonta perp/ana Cnnra.d. Dorsal valve, showing shell in nepion'ic 

 stage, the fold, and the origin of the striae. X ^6. 



Fig. II. Tiie same species. Outline of the ventral valve, showing the acuminate 

 cardinal extremities. X 3- 



Fig. 12. The same species. Interior of ventral valve, to show the diductors and 

 the two pairs of adductor scars. X !• Compare with Figs. 14 and 17. The speci- 

 mens are in the writer's collection, now deposited in the Carnegie Museum. 



Nepiouic Stage. — The shell, in the nepionic stage, is convex in 

 both valves, nearly as long as wide, and with a hinge width about as 

 great as the width below. In the dorsal valve there is a fold which 

 extends nearly to the front of the shell. Otherwise the valves are 

 smooth. The dimensions at this stage are : in one shell, .56 mm. long 

 and .64 mm. wide; in another, .60 x .72 mm. (Fig. 10). 



Changes Dining Development. 

 Outline. — In early neanic stages, the width of the hinge becomes 

 greater than the width below, and, during all the adolescent period, 

 the shells are strongly alate and the hinge width is frecjuently two and 

 a half or three -times the length of the shell (Fig. 11). In the adult 

 stages the hinge width is still the greatest width of the shell, but the 

 cardinal extremities are not far extended. In one young individual, 

 with a width of 7 mm. on the hinge line, each extremity forms a 

 spinelike extension 1.5 mm. long, thus making up nearly one half the 

 total width. Other specimens 10 mm. w ide at the hinge, have alate 

 extremities, each 2 rnm. in length. In computing the index, the 

 width below the hinge was taken and it was found that, apart from 

 the extensions of the cardinal angles, there is very little change in form 

 through life. The older shells are a little longer, in proportion to the 

 width, than the young ones. The index, in neanic stages, is about 

 1.45 to 1.30 mm. and, in the adult, 1.35 to 1.20 mm. 



