OF CONCHOLOGY. 199 



bles Isocardia in the obliquity of its outline, in its anterior, 

 spiral beaks, and even in the hinge. The large tooth of the 

 right valve has its analogue in the much smaller, though simi- 

 larly shaped, anterior tooth of the right valve of Isocardia. The 

 other cardinal teeth of Isocardia are simply suppressed, and the 

 difference in the posterior laterals is one not of structure but 

 merely of degree. 



The positions of the muscular scars and the form of the pal- 

 lial margin are nearly identical in both genera. 



P. obliqua, Gabb. — PI. 16, fig. 5, a — e. 



Description. — Shell small, oblique, very inequivalve, thick ; 

 beaks anterior, terminal, spiral ; valves convex, right valve near- 

 ly twice as high as the left ; cardinal margins arching from the 

 beaks to the posterior basal angle with a broad, irregular and 

 very variable curve ; anterior end and base continuously curved, 

 the anterior end retreating from the beaks, or occasionally very 

 slightly produced beyond them. Surface marked only by lines 

 of growth ; the shell was apparently covered by an epidermis, 

 when alive. A distinct posterior umbonal ridge runs from the 

 beaks to the posterior basal angle, and in some specimens a sec- 

 ond one occurs between this and the cardinal margin ; the latter 

 is best marked, usually, in the right valve. There is no lunule, 

 but in the left valve the anterior face is produced laterally much 

 more than in the right, to a point corresponding with the margin 

 of the deep pit in the hinge, where it is suddenly truncated at a 

 right angle, thus making the anterior or lower side of the pit 

 very much higher than the opposite margin. Anterior muscular 

 scar deeply impressed ; posterior faint ; pallial line slightly 

 bent up just before reaching the posterior scar. 



Dimensions of right valves : 



Length, -43 in. Width, -35 in. Height, -16 in. 



" -44 in. " -40 in. " -18 in. 



" -38 in. " -30 in. " -15 in. 

 Left valves : 



Length, -40 in. Width, -32 in. Height, 40 in. 



" -38 in. « -28 in. " -08 in. 



" -38 in. " -30 in. . " -09 in. 



P. tenua, Gabb. — PL 16, fig. 6, 6a. 



Description. — A second form of the genus is represented by a 

 single left valve, more than twice as large as the other specimens.' 

 It is proportionally much thinner, and differs very materially 



