368 TWENTIETH REPORT ON THE STATE CABINET. 



BRACHIOPODA. 



GENUS OBOLUS (Eichwald). 



OBOLUS CONRADI (n. s.). 



PLATE 13 (4), FIGS. 1 & 2. 



Shell depressed orbicular or subdiacoid ; width usually greater than 



the length, gibbous in the middle and compressed towards the margins. 



Dorsal valve more convex than the ventral. Surface unknown. 



The rostral portioii of the valves is extremely thickened, for muscular 

 attachments ; and this area extends in an elevated plate or transverse sep- 

 tum which becomes free at its anterior margin, except where it is supported 

 in the middle by a vertical septum. The interior surface of this plate, 

 towards the antero-lateral margins, is marked on each side by a rhomboidal 

 muscular scar, varying somewhat in the two valves, and in different indi- 

 viduals. On each side and just without the upper or posterior lateral 

 margins of this plate, there is a depressed oval or reniform muscular scar, 

 varying in character and area in the two valves. In the dorsal valve, and 

 probably in the ventral valve also, there is an elongate ovate imprint, 

 extending from a narrow point, at what may be regarded as the extremi- 

 ties of the hinge line, close to the cardino-lateral margins, and gradually 

 expanding below, reaching nearly half way to the anterior margin of the 

 shell. This transverse plate in the ventral valve sometimes shows muscular 

 markings just below the rostral area as well as anteriorly. The cast of the 

 rostral cavity is small and neatly defined, with marks similar to those of 

 dental lamellae ? along the cardinal slopes. The cast of the dorsal valve 

 shows the impression of a broad shallow spoon-shaped plate, with the 

 median septum extending nearly to the base of the shell. 



The figures are given from casts of the dorsal and ventral valve. The 

 impressions from these were unfortunately omitted, but will be given in a 

 plate accompanying the next Report on the State Cabinet. 



Some ten years since, having these fossils under consideration, I pro- 

 posed a new generic name for them ; but sending drawings to Mr. David- 

 son, he gave me the opinion of himself and Mr. Woodward that they 

 belonged to the G-enus Obolus. There are certain points of difference, 

 however, which I have been inclined to believe are of generic importance, 

 and I am not entirely satisfied in referring them to that genus, as illustra- 

 ted, though closely allied to it. 



In the mean time, Mr. Billings has published a Genus Trimerella, 

 illustrating it by a specimen from the G-uelph limestone. Now the fossils 

 under consideration, in the earlier stages of growth, have the transverse 

 plate but partially attached at the sides ; and the processes shown in Mr. 

 Billings's figures are sometimes slightly simulated by the casts of the 

 spaces on each side of the median septum, which extend between this 



