207 



The vascular trunks extend forward in a regular arch from the middle 

 of the valve a little within the flattened margin, which is creased trans- 

 versely by about a score of closely set parallel grooves. In the anterior 

 third these give place to grooves that are at right angles to the margin ; 

 these correspond in course to the faintly impressed sub-parallel 

 grooves that extend from the front margin across the middle of the valve 

 to the visceral callus. Faint traces of branches of the vascular trunks 

 are seen on the slopes of the valves in the anterior half. 



The dorsal valve is of an oval form. It is strongly arched down in the Dorsal valve, 

 posterior half, but less so on the anterior slopes. Interior. This shows 

 at the cardinal lines a depression in which are a pair of circular pits, due 

 to the cardinal muscles. Between these pits, on the axial line, is a small 

 pit from which two sharp low ridges run forward ; at one-third from the 

 back of the valve there is a minute scar between these ridges ; and outside 

 of them in the posterior half of the valve are the large oval prints of the 

 central ( " h " ) muscles ; these are set somewhat diagonally to the axial 

 line, having the fronts turned outward. At the anterior ends of the 

 median ridges are the small scars of the anterior lateral ( " j " ) muscles. 

 Faint diverging ridges extend from the umbonal cavity toward the lateral 

 margins of the valves ; at one-third from the back, partly on and partly 

 outside the ridges are the large but rather faint imprints of the posterior 

 lateral muscles. 



This valve, like the ventral, has flattened margins on which are imprint- 

 ed minute, closely set, transverse grooves. 



Sculpture. The sculpture of the true outer surface of this species is Sculpture, 

 not easily found ; it is imprinted on a thin calcareous, fibrous layer, which 

 is usually broken away, revealing the next layer of the shell. The outer 

 layer is crossed transversely by closely set strise, forming ridges, of which 

 there are about nine or ten in the space of a millimetre ; some of these 

 ridges have cross strise at intervals ; others anastomose, and all have a 

 roughened surface ; the ridges have a waving course over the middle 

 third of the shell, but elsewhere are comparatively straight. 



Beneath the outer shell is a corneous layer -whose sculpturing conforms 

 to that of the outer layer, but the strise are wider and the intervening 

 ridges narrowed ; this layer has a shining surface. Beneath this is a 

 third layer on which the striae run in an opposite direction from those of 

 the one above, the sculpturing, especially along the central part of the 

 valve, consisting of strise radiating from the umbonal region to the front 

 margin ; these are crossed at intervals by undulations of growth concen- 

 tric to the umbo ; on the inside of this layer are impressed the surface 

 markings of the interior of the valves. 



