on the Genera Orbieula and Crania of Lamarck. [69 



with the foregoing, and have been elicited by the smie dis- 

 covery. 



From an attentive examination of two specimens of the shell 

 described and figured in the Linnean Transactions, under the 

 name of Patella distorta, I was led to suspect that it might be ;t 

 bivalve shell, and probably related to the genus Orbieula, and 

 belonging to the family of Terebratulidea. 1 have it now in my 

 power to show that this suspicion is verified: for Mr. Bullock 

 has obligingly communicated a stone from one of the Shetland 

 islands, to which are attached several specimens of this shell 

 (Patella distorta). Upon lifting one of these, I was not a little 

 pleased to discover, in a dry state, the two fringed arms or fcen- 

 tacula common to, and characteristic of, the Terebratulidea ; and, 

 firmly adhering to the stone, another valve, white, extremely 

 thin, except at its edges, and having four muscular impressions 

 corresponding with those of the upper brown valve ; but two of 

 these muscular impressions are certainly so near together, that I 

 do not wonder that, upon a slight examination, Lamarck should 

 have described the genus Crania as having in the lower valve 

 three oblique perforations. It therefore appears that this shell, 

 instead of being a Patella, may properly be considered a bivalve 

 shell, and that it belongs to Retzius's genus Crania, of which 

 Lamarck gives the following characters : 



" Coquille composee de deux valves inegales, dont l'inferieure 

 presque plane et suborbiculaire est percee en sa face interne 

 de trois trous obliques et inegaux. La superieure, tres con- 

 vexe, est munie interieurement de deux callosites saillantes." 



These characters however do not appear to me quite satisfactory. 

 I would suggest the following as an amended generic character. 



CRANIA, k 

 Bivalve, inequivalve, nearly orbicular, compressed, fixed ; upper 

 vol. xin. 3 P valve 



