320 PROCEEDINGS OP THE ACADEMY OP 



bonal and antero-central regions ; umbonal slopes ranging at an an- 

 gle of about fifty degrees below the hinge line, and broadly rounded; 

 hinge line straight, very nearly or quite equalling the greatest an- 

 tero-posterior diameter of the valves, and ranging nearly at right 

 angles to the anterior side of the same ; posterior alation very 

 large, not separated from the swell of the umbonal and central 

 regions by any defined sulcus, sliglitly rounded at its immediate 

 extremity above; posterior margin faintly sinuous for a little 

 below its intersection with the hinge margin above, thence sloping 

 forward and downward, and finally rounding into the regularly 

 rounded base ; anterior side more or less concave, and nearly 

 vertical above, but rounding regularly into the base below ; beaks 

 terminal, rather pointed, rising little above the hinge line, and 

 directed a little obliquel3'^ upward and forward, with more or less 

 inward curvature. 



Surface ornamented by about twenty-four to twenty-eight sim- 

 ple, strong, radiating costie to each valve, that are nearly equal in 

 breadth to the furrows between ; those on the central portions of 

 the valves passing nearly straight from the beaks obliquely to 

 the posterior basal margins, those on the anterior side curving 

 more or less forward below, and those near the cardinal margin 

 curving a little upward behind. Crossing all of these cost8e,and the 

 furrows between, are numerous fine crowded lines, and, at regular 

 distant intervals, a few strongly defined imbricating marks of 

 growth that curve parallel to the basal and posterior margins. 



Height, 2.30 inches ; breadth, 2.20 inches; convexity, about 0.80 

 inch. 



This species was referred b}^ Mr. James, with a mark of doubt, 

 to Megaptera Casei of Meek and Worthen ; but, after a compa- 

 rison of these shells, I can find no reason for doubting that they 



class, nearly all agreo that only the name first given can stand ; but, where 

 they belong to different classes or subkingdoms, some would retniu both 

 names, while others would change the later name, even where one of the 

 genera belongs to a diiferent class, subkingdom, or kingdom of nature. If 

 it should be thought desirable to substitute another name for this group, as 

 typified by 31. Casei and the species here described, I Avould propose to call 

 it Opisthoptera. As yet very little is known in regard to the hinge of these 

 shells ; and consequently we have not the means of determining whether 

 they should be ranged as a subgenus under AmhonyrJiia^ or as a distinct 

 genus, though I at present incline to the former opinion. 



[February 13, 



1 



