70 [Senate 



foramen at the base of the area are but slightly thickened, but little produced, and 

 have scarcely the appearance of the teeth of Orthis. 



In Orthis pyramidalis, 1 have not been able to obtain separated valves ; but so 

 far as can be ascertained, we have a similar internal structure. In the species from 

 Tennessee the same characteristics appear, except, perhaps, that the specimens 

 seen do not exhibit the bifurcation of the median septum of the dorsal valve. 



For these forms I propose the generic designation of Skenidium, in allusion to 

 the form of the ventral valve. 



GENUS SKENIDIUM(n.g.). 

 [ Gr. I,Krjvi6tov, tentoriolum, a little tent.] 

 Shell subpyramidal, somewhat semicircular, with or without median sinus and 

 elevation. Area large, triangular, divided by a narrow deltoidal foramen which 

 is sometimes closed at the summit by a concave deltidium. Valves articulating 

 by teeth and sockets, which are often obscure or obsolete. Dorsal valve flat, or 

 varying from depressed convex to concave; beak entire, or indented by the fo- 

 ramen. Cardinal line straight, and usually equalling the width of the shell : 

 cardinal plates broad and well developed, marked by the imprints of the pe- 

 duncular muscles, and produced in the middle in a pointed process. The cardinal 

 process extends as a median septum through the length of the shell, and may 

 be simple or divided at its extremity. Ventral valve elevated, subpyramidal; 

 beak straight or slightly arched. Muscular impressions undetermined. Exterior 

 surface covered with radiating striae. 



The species which I propose to include under this genus have the general aspect 

 of Orthis, except in the extreme elevation of the ventral valve, which gives a 

 pyramidal aspect to the shell when resting upon the dorsal valve. In one species, 

 S. insignis, the median septum is clearly bifurcated near the extremity, and 

 expanded in width so much that it divides the interior cavity. In a species from 

 Tennessee, for which I am indebted to the kindness of Prof. Safford, the septum 

 appears to be simple, and extends to the anterior margin of the shell. The interior 

 of S. pyramidalis of the Niagara group has not been determined, and it is united 

 with the others from its exterior form and surface markings . 



Geological distribution of the species. Of the three species known, one occurs 

 in the Lower Silurian strata of Tennessee, one in the Niagara group, and one in 

 the Lower Helderberg group. 



The accompanying figvu-es illustrate the characters given in the descriptions above. 



Fig. 1. Front view of a specimen of Skenidium insigniSj enlarged. 

 Fig. 2. Cardinal view, still farther enlarged. 

 Fig. 3. Exterior of the dorsal valve. 

 Fig. 4. Interior of the ventral valve. 



Fig. 5. Interior of the dorsal valve, showing the wide hinge -plates and the bifurcating median 

 septum*. 



* As this description is going to the press, my attention is called to the Genus Ehynchora of 

 Dalman, as illustrated by Davidson in the species R. spatulata of Dalman. The interior of 

 the dorsal valve of that species, particularly in the median septum, bears much resemblance to 

 the dorsal valve of Skenidium; but I cannot suppose there will prove to be generic identity. 



