Tkopidoi.kptus Fauna at Canandai(;ua Lakk, N. \'. 85 



The index of the nepionic shell is about i.io to 1.15 ; of the 

 neanic .79 to .95, and of the adult .76 to .90. 'I'he average for ten 

 specimens in neanic stages is. 86, and of a similar number of adults, .83. 



The shell substance is impunctate. 



Muscle Scars. — The muscular area is roughly shield-shaped with 

 the broad end directed backward. The front end is about the middle 

 of the valve. The area is very small, as compared with the size of 

 the valves. In a s|)ecimen 3.93 mm. long and 3.20 mm. wide the 

 muscular area is 1.06 mm. long and .86 mm. wide. In a smaller 

 specimen, 1.5 x 1.3 mm , the area is .5 x .5 mm. 



In both valves there are narrow transverse scars on the anterior and 

 posterior borders of the area. In the ventral valve, behind the an- 

 terior transverse scar, are two large oval adductors. Their anterior 

 ends nearly meet, and the posterior ends are widely separated. Back 

 of them, at the corners of the shield, are two small round scars, and 

 there is a double scar on the median line. 



In the dorsal valve the anterior adductor scars are narrow and par- 

 allel. Behind them are two smaller scars and between them a median 

 scar. 



Pholidops oblata Hall. (Plate II.) 

 Hall, Pal. N. V., IV., 1S67, p. 414, pi. 3, fig. 10. 



Of this species there are about 200 specimens in the collection. 



The smallest is 1.23 mm. long and i mm. 



wide. The largest is 7.8 mm. long and 7 



mm. v/ide. A specimen of this species 



described by Hall is about twice the size of 



the largest shell in our collection. It is 



15.5 mm. long and 14 mm. wide. 



Nepionic SJiell. — As in P. liainilionicc ,^ r,, ,■ j , , , j 



^ riG. I. Pholidops oblata 



the nepionic shell is oval, broader than Hall. Nepionic and early ne- 



long, convex, and with a slightly curved anic stages, drawn from a dorsal 



hinge line. The dimensions of the smallest l^eak. They might be termed 



shell of this stage are .18X.20 mm.; or ^"^^ Crania ^.n6. Pholidops ham- 



^11 ^ /T-- \ iltoni(e stsiCQS. V 30- 



the largest .25 x -27 mm. (Fig. i). & /\ j 



Changes During Development. 



After the nepionic stage the growth is more rapid in front than at 

 the sides and an elongate oval form is produced when the shell is 

 about .6 mm. long. The beak is then marginal and the greatest 



