NATURAL SCIENCES OP PHILADELPHIA. 167 



Description of a new genus of the family DEEISSENID^. 



BY T. A. CONRAD. 



MYTILOPSIS. 



Shell mytiliform, attached by a byssus ; hinge with a septum, beneath which 

 on the cardinal side is a triangular cup-shaped process; cartilage groove 

 rather deep. 



Mytilus LEUcoPHiEATUS, Con. Joum. Acad. Nat. Sciences, vol. vi. p. 263, pi. 

 11, fig. 13. 



This singular bivalve inhabits the rivers of "Virginia and probably further 

 south, where the water is brackish, resembling Cyrena in that respect, and it is 

 found in great abundance attached by its byssus to Ostrea Virginiana. The 

 hinge resembles that of Sepiifer and Breissena, with the addition of a singular 

 cap-shaped, thin, white appendage, which projects obliquely towards the cavity 

 of the valves. Another species inhabiting St. Domingo has been figured and 

 described by Recluz as Dreissena Domingensis. 



Notices of some Bemains of Extinct Fishes. 

 liY JOSEPH LEIDY, M. D. 



1. Hadbodus priscus, Leidy. 



The genus and species are founded upon the fragment of a bone with two 

 teeth, apparently of a Pycnodont fish allied to Placodus, obtained by Dr. William 

 Spillman, from a cretaceous deposit in the neighborhood of Columbus, Mis- 

 sissippi. 



The fragment of bone is about IJ inches in depth and breadth, and about 

 ;( of an inch in thickness; is convex on the outer side ; and presents large re- 

 serve cavities on the inner side at the base of the two teeth which are coossified 

 with one of the borders of the bone. The teeth present a remarkable resem- 

 blance to premolars of a pachydermatous mammal. They are quadrate, and are 

 about as broad as they are high, and about half the thickness. They are bilobed 

 at the triturating surface, which slopes inwardly ; and are invested with smooth 

 enamel, which extends twice the depth externally that it does internally. The 

 two teeth differ a little in form and size. Their height externally is 8 lines ; the 

 breadth of one 8 lines ; of the other 7 lines; and the thickness of both is 4 lines, 

 except the distal lobe of the larger tooth which is 5 lines. 



'I. Phasganodus DIR0S, Lcidy. 



This genus and species are founded upon a much mutilated dental bone with 

 teeth imbedded in a hard mass of sandstone, discovered in Nebraska, by Dr. F. 

 T. Hayden. The specimen I suspect to have been obtained from a cretaceous 

 deposit. 



The dental bone in its perfect condition has been about 6 inches in length, 

 and 2 inches in depth posteriorly. To the dentary border, so far as can be as- 

 certained, there have been six large, coossified, sabre-shaped teeth. The trench- 

 ant border is directed outwardly ; and the obtuse, inner, concave border is- 

 longitudiually ridged. The first tooth visible is about | of an inch from the end 

 of the jaw, and has been about 1] inches long. The second tooth was situated 

 about 1 inch posterior to the first^ and was about f of an inch long. The third 

 tooth, the best preserved in the specimen, about h an inch behind the second, 

 has a strong conical base, and it is 10 lines long". The succeeding teeth de- 

 crease in size, and are at irregular distances apart. The specimen I suspect to 

 belong to a scomberoid fish allied to Enchodus. 



3. TuRSEODUs ACUTus, Leidy. 



This genus and species are founded upon a left dental bone with teeth pro- 



1857.] 



