FOSSILS OF THE NIAGARA GROUP. 375 



BRACHIOPODA. 



GENUS OBOLUS, Eichwald. 



Compare TRIMERBLLA, Billings. 



Obolus GONRADI, N. S. 

 plate xiii, figs. 1, 2; plate xxv, figs. 1, 2, 3. 



Shell depressed orbicular or subdiscoid ; 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 portion of the valves is extremely thickened, for muscular 

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

 septum which becomes free at its anterior margin, except where it 

 is supported in the middle by a vertical septum. The interior sur- 

 face 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 individuals. 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 extremities 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 lamellge C?) along the car- 

 dinal 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 on Plate xiii are given from casts of the dorsal and ventral 

 valve. The impressions from these are given on Plate xxv of this edition 

 of the Report. 



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

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

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



