CONTRIBUTIONS TO PALiEONTOLOGY, 65 



and striated on their exterior faces, but not '* grooved or bidentated towards 

 " the extremity of their outer surface" as in Streptorhynchus. Here 

 again, however, it becomes necessary to distinguish the cardinal process of 

 Streptorhynchus from that of Strophodonta ; for the Ij^tter is a bi- 

 furcating process, and the divisions are bidentate or grooved on their outer 

 face. Aside from the crenulated hinge-line, which is usually a distinguishing 

 feature, the cardinal process of Strophodonta is more divaricating than 

 that of Streptorhynchus, and the extremities of the divisions are thick- 

 ened ; while in the latter, they are often, if not always, more slender, and 

 sometimes flattened towards the extremities. In Streptorhynchus, the 

 cardinal process is supported laterally by socket-plates, which are often 

 strongly developed ; while in typical species of Strophodonta, these 

 socket-plates are wanting or rudimentary. 



The muscular impressions in the dorsal valve of Streptorhynchus are 

 more like Orthis ; while in the ventral valve we have, to some extent, the 

 varieties of form and proportions of muscular impressions observed both 

 in Orthis and Strophomena. In one species of Carboniferous age, there 

 is a strong elevated septum dividing the muscular impression of the ventral 

 valve throughout its entire length, becoming thicker above, and, reaching 

 the deltidium at the apex, is conjoined on either side with the bases of the 

 dental lamellae. 



One of the most extreme examples, perhaps, is the Orthis [Streptorhyn- 

 chus) hipparionyx of the Oriskany sandstone. From external characters, 

 and from the remarkably large flabelliform muscular impression of the 

 ventral valve, it has been regarded without hesitation as an Orthis.* It 

 is one of those forms termed resupinate ; the dorsal valve being unusually 

 convex, and the striae curving upwards to the hinge-line in a most extreme 

 degree. The ventral foramen is closed by a deltidium, and the interior of. 

 the dorsal valve is furnished with a large bifurcating cardinal process, each 

 division of which is deeply grooved on its outer face, and the extremities 

 bidentate. There is a median ridge of moderate strength and strongly ele- 

 vated socket-plates, as shown in the accompanying figure. 



Fig. 6. 



Fig. 6. Interior of the upper part of the dorsal valve of Streptorhynchus (Orthis) hippari- 

 onyx : from s^ specimen of median size.f 



* The Genus Hipparionyx, proposed by Mr. Yanuxem, inclnded ^trypa reticularis 

 as well as this species. 



t For full illustrations of this species, see Palaeontology of New-York, Yol. iii. pa. 

 407, pi. 89 & 90. 



[Senate, No. 11^.] 9 



