RHYNCHONELLA. 91 



Museum of the Geological Society. It is, likewise, probable that the vertical range of 

 this species extended beyond the limits here traced : some specimens from the Cornbrash 

 being undistinguishable from certain varieties of the shell under consideration. 



On the Continent, it is abundantly distributed in the Great Oolite of Ranville, &c, in 

 Normandy. From B. concinna it seems distinguished by its dimensions, larger, and 

 deeper plaits ; the deltidium surrounding and separating the foramen from the umbo, 

 which character appears to have been noticed by Mr. Sowerby, since he gives a separate 

 figure of this portion of the shell : the hinge margin indenting likewise more the smaller 

 valve than in concinna, it therefore could not be a young state of this last-named shell, as 

 erroneously supposed by M. D'Orbigny. 1 In the young state, both B. concinna and 

 obsoleta are quite distinct ; the last being more elongated and deeply plicated, while in con- 

 cinna the shell is transversely compressed, and the plaits are smaller and more numerous. 

 Professor Bronn 2 considers B. obsoleta a synonym of B. tetraedra, but I believe the two 

 species can be easily distinguished. Sowerby's figures both of B. concinna and obsoleta are 

 very unfortunate, as well as those of tetraedra and media, from the fore-shortened aspect 

 selected for illustration. 



Plate XVII, fig. 1. From the Bradford Clay, in the collection of the Geological 



Society, being the largest specimens I have seen of the 

 species. 

 ,, figs. 2, 3. Young shells, from Bradford. 



,, fig. 4. Interior. Illustrating the two short shelly lamella a, to which 



were attached the free fleshy arms ; b, are the muscular 

 impressions. 

 ,, fig. 5. The beak, foramen, and deltidium (enlarged). 



85. Rhynchonella subobsoleta, Dav. Plate XVII, fig. 14. 



Diagnosis. Shell inequivalved, circular, semi-globose, nearly as broad as long ; beak 

 moderately produced ; foramen circular, entirely, or otherwise surrounded by the deltidial 

 plates, a small portion being generally completed by the umbo ; false area not very well 

 defined, the hinge margin not much indenting the smaller valve. Surface ornamented by 

 a variable number of large costse, from nineteen to twenty-one or two on each valve ; the 

 mesial fold not much produced nor always distinct, formed of from four to six plaits ; sinus 

 shallow. Structure unpunctuated ; length 12, width 12, depth 9 lines. 



Obs. This shell is always much smaller than either B. obsoleta or concinna, rarely 

 attaining the dimensions given above, it belongs to the middle division of the Inferior 

 Oolite of Minchinhampton, according to Messrs. Lycett and Woodward. It possesses 



1 Prodrome, vol. i, p. 315. 



2 Index Pal., pp. 1243 and 1253. 



