202 



Description 

 of var. lens. 



Sculpture. 



Shell substance calcareo-corneous. A broadly ovate form with rather 

 thin, smooth valves, having flattened lateral slopes in the ventral valve, 

 and being somewhat tumid toward the umbo in the dorsal valve. 



Ventral valve rather blunt at the umbo, whence for about one-third of 

 its length the curve of the margin is somewhat straightened ; for the rest 

 of the border it is regularly rounded to the front ; the greatest width is 

 a little in front of the mid-length. The umbonal ridge extends about half 

 of the valve, whence to the hinge the sides of the valve are flattened ; in 

 front of the middle of the valve the slopes are evenly but flatly arched 

 down to the margin. Interior. The position of the central group of 

 muscles is within the posterior third of the valve, and the position of the 

 laterals is indicated by a bounding ridge ; these features are very faintly 

 marked. 



The dorsal valve is broadly ovate, and its slopes are more strongly 

 arched in the posterior half than elsewhere, otherwise it is like the ven- 

 tral. Interior. This has a sharp, low septum for half its length, and on 

 each side a parallel ridge, extending to the middle of the valve ; at half 

 the length of these ridges are small lenticular scars, and at their outer 

 ends the group of central muscles. The lateral muscles are opposite the 

 middle of the median ridge. 



Sculpture. The outer crust in this species is normally smooth in 

 appearance, but is beset with minute pits, and has a very minute con- 

 centric and radiating striation. The sculpturing of the layer beneath 

 has impressed itself on the outer layer in different parts of the surface ; 

 at the sides and in front we find concentric ridges, and in the middle 

 third the imprint of the vascular striae that run toward the front margin. 

 These markings are much more distinctly shown on the under layer. 



Size. Length of the ventral valve, 16 mm. ; width, 13 mm. The dor- 

 sal valve is 1 mm. shorter than the ventral. 



Horizon and locality. Thin calcareous layers in the flags of Div. 3a 

 at McAdam shore, Escasonie, Cape Breton. The shell in these layers are 

 freely intermingled with small lumps and particles of calcium phosphate. 

 The phosphate lumps are frequently moulded on the shells, or entirely 

 enclose them ; though some shells are enclosed in the phosphate, others 

 are free, and with fragmentary shells are mingled with the sand. Other 

 masses of the phosphate are entirely free of the shells, and are smooth 

 and shining, as though rolled on the beach ; yet the flat, oval, or rod-like 

 pieces of the phosphate seem the natural form which the substance as- 

 sumed when in a gelatinous condition. Probably the formation of the 

 phosphate was cotemporary with the entombment of the shells. 



