NATURAL SCIENCES OP PHILADELPHIA. 437 



rounded, a little convex on its posterior outline ; less elevated than the glabella. 

 Anterior slope in advance of the glabella, less than half as wide as the length 

 of the latter, and provided with a distinct, rounded transverse furrow, which 

 passes around slightly in front of the middle of this space parallel to the an- 

 terior and lateral margins. Cheeks convex, and sloping towards the lateral 

 and antero-lateral border. Surface and facial sutures unknown. 



Length, measuring from the posterior side of the neck segment to the front 

 margin, 0*57 inch ; greatest breadth measuring across at the posterior extremi- 

 ties of the cheeks, 0-87 inch ; height, 0'31 inch. Length of glabella, including 

 the neck segment, 0-40 inch ; breadth of glabella, 0-35 inch. 



The only specimen of this species we have seen is a cast, retaining none of 

 the shell, and of course giving no idea of the nature of the external markings, 

 if there were any. Nor is it in a condition to enable us to determine whether 

 or not the posterior lateral extremities of the buckler are pointed, though they 

 appear to be. At a point nearly opposite the middle of the glabella, there is 

 on each cheek less than half way down the slope, from the furrow between 

 the cheeks and the glabella, what appears to be some remains of a small eyes, 

 though the specimen being unfortunately a little defective here on both sides, 

 the exact nature of these prominences cannot be clearly made out. 



We sent drawings of this species to Dr. Shumard and Mr. Billings, both of 

 whom write that it is specifically distinct from any of the forms described by 

 them from rocks of the same age in Cana la and Texas. We had been inclined 

 to refer it to Mr. Billings' genus Bathyurus, but have concluded to place it pro- 

 visionally in Barrande's genus, Arionelius, until better specimens can be ob- 

 tained, showing more satisfactorily its generic characters. We have been led 

 to do this, by the proportionally small size of some imperfect caudal shields, 

 probably of the same species, embedded in the same slab; in which respect 

 it would seem to agree more nearly with Barrande's genus, than with Bathyu- 

 rus, the type of which, Mr. Billings writes, has a caudal shield nearly one-third 

 as large as the entire animal. 



Both of these genera seem to be very closely allied to Crepicepalus of Owen, 

 but as that genus is not very clearly defined, and was based upon specimens 

 probably belonging to more than one group, it will perhaps not be generally 

 adopted. Our species is certainly quite closely allied to some of the specimens 

 figured by Dr. Owen under that name. 



Locality and position. Same as last. 



JURASSIC SPECIES. 



LA MELLIBRANCH1A TA. 



Genus GRYPHvEA, Lamarck. 



Gryph^a calceola, var. Nebascensis. 



Grypheea calceola, Quenstedt, Handb. Petref. tab. 40, fig. 29-31. 



GryphiMu calceola, Quenstedt, Der Jura, tab. 48, fig. 1-3. 



Gryphaza calceola ? Meek, Capt. Simpson's Rept. Utah Expedition. 



Amongst the collections brought in by Capt. Simpson's Expedition, from the 

 Jurassic beds at the Red Buttes, on the North Branch of the Platte River, 

 there are specimens of a small irregular oyster-like Grypheea, which were re- 

 ferred with doubt by one of the authors to Gryphma calceola, of Quenstedt. 

 These specimens generally have the whole umbo completely truncated by the 

 scar of attainment, so as to present none of the characteristic features of the 

 genus Grypheea. Some of them, however, have the beak less distinctly trun- 

 cated, and a few have it nearly entire, pointed, and incurved as in the true Gry- 

 pkceas. Still none of these specimens are one-fourth as large, nor have any of 



1861.] 



