264 ROYAL SOCIETY OF CANADA 
straight, or very gently convex, for about two-thirds of the width in the 
middle ; anterior angles rounded ; sides straight or slightly convex and 
subparallel until within two-thirds, or one-fourth the length from the 
back, then converging to the apex, where they form an obtuse angle which 
varies from 100 to 110 degrees. This valve is generally very convex, 
sometimes almost hemispherical ; the outline on a side view is rather 
abruptly elevated in the rostral third, depressed convex for a short space 
in the middle and then more gently descending to the front margin. 
Most of the specimens of this valve are eight or nine lines in length and 
about the same in width. 
“ The shell which is supposed to be the ventral valve of this species is 
gently convex, with usually a somewhat flattened space extending from 
the front margin upward toward the beak. The apical angle appears to 
be from 90 to 100 degrees. Shell very thick, of a lamellar structure, dark 
brown or nearly black, and sometimes when exfoliated, of an ashy gray 
colour. Surface with a number of obscure undulations of growth, and with 
fine longitudinal striæ about ten in the width of one line.” 
In the examples which the writer has seen the following features 
were observed : 
The mould of the interior of the dorsal valve shows heavy muscle- 
scars. At each end of the hinge line is a small umbo-lateral scar ; from 
these scars a crescentic ridge extends forward on each side of the valve 
about two-thirds of its length, and terminates at a small scar ; within the 
bow of the crescent and about midway of its length is a large oval sear. 
The central scars as a group are in front of the midlength of the valve ; 
of these the lateral scars are equidistant from the beak and front of the 
valve ; the central scar is faintly double and is nearly three-quarters the 
length of the shell from the hinge; behind it is an oval scar, elongated: 
and having a septum or thin plate on the median line of the valve. 
Vascular trunks extend forward on each side of the valve, from the 
middle almost to the front margin ; from these trunks about haif a dozen 
branches turn inward toward the visceral cavity, and more numerous 
branches outward toward the margin of the valve. 
The ventral valve also has strong muscle-scars. There is no pedicle- 
groove, but a small rhombic ridge extending into the deltidial area may 
indicate a pedicle-pit; behind this on each’ side are small scars (of the 
cardinal muscle ?) 
The lateral muscles of this valve are more approximate than those 
of the dorsal, and the crescent is not so distinct ; the umbo-lateral scar is 
near the margin and is separated from the other two laterals which form 
a sigmoid curve and are in contact at theirends. In the central group of 
muscles the large paired scars are smaller, and in some examples closer 
together than those of the dorsal valve, and between them lies a median 
