18 REPORT 1844, 



Perforated. 



Obovata. 



Ornithocephala. 



Ovata. 



Perovalis. 



Sphasroides. 



Not Perforated. 



Variabilis. 

 Subplicata. 

 Tetraedra. 

 Wilsoni. 



Tliis list will enable any one conversant with the genus to see, that, with 

 scarcelj" an exception, the perforated species are smooth, or but slightly pli- 

 cated, not exceeding in their plication the Terebratula caput serpentis, which 

 is, I believe, the most plicated of the recent species ; whilst the non-per- 

 forated species are deeply plicated*. Besides the species named in this list, I 

 have examined about ten other species of non-plicated Terehratulce, whose 

 names I was unable to ascertain ; they all agreed with the other non-plicated 

 species, in the possession of the perforations. 



43. Among the genera most nearly allied to Terebrahda, I have usually 

 found a similar variation. Thus, Orthis canalis and Orthis (Spirifer, Phil.) 



Jiliaria present exactly the same structure as the perforated Terehratulm ; 

 whilst Orthis hemiproiiites, Orthis resupinata, and another species from the 

 Silurian formation, Ohio, are destitute of perforations. 



44. In Spirifer, again, the perforations are present in some of the sj)ecies, 

 and absent in others. For want of good specimens I have not been myself 

 able to examine many species of this genus ; but I have found the perforations 

 very well marked in Spirifer Walcotii of the Lias, whilst they are absent in 

 Sjnrifer cuspidatus and another Mountain Limestone species, and in a species 

 from the Devonian formation at Hudson's Bay. I learn from Mr. Morris, 

 that he has remarked the punctations in the Spirifers of the Silurian and 

 later secondary strata, but not on those of the mountain limestone ; which 

 circumstance he attributed to the metamorphic condition of the shell in the 

 latter. I am satisfied, however, that such is not the case ; since, although the 

 structure of the shell is often obscured by this action, I possess sections in 

 which it is extremely well preserved, and in which there is an evident absence 

 of the perforations. 



45. In no Ati~ypa, however, have I met with perforations. The species I 

 have examined are Atrypa ojffinis, A.pugnvs, A. lineata, A. galeata, and a 

 crag species closely allied to Terebratula psittacea, if not identical with it. 



46. In Pentamerus Knightii I have found the structure characteristic of 

 the group, but without perforations. 



47. The structure of the shells of Lingula and Orbicula is equally peculiar, 

 but very different from that which has been now described. These shells are 

 almost entirely composed of laminte of horny matter, which are perforated 

 by minute tubuli, closely resembling those of ivory in size and arrangement, 

 and passing obliquely through the laminae (fig. 22). Near the margin of the 

 shell, these tubuli may be seen lying nearly parallel to the surface. 



X. PlacunidcB. 



48. This family has been separated by Deshayes from the OstracecE, and con- 

 stitutes, according to his views, " a descending and lateral line, really inter- 

 mediate between the ordinary Bivalves and the Brachiopoda." The propriety 

 of such an arrangement is completely borne out by the microscopic structure 



* There are one or two apparent exceptions to this, as the case of the Terebratula sub- 

 plicata, in wliich the plications are veiy slight ; hut this is thought hy Mr. Morris to be the 

 young of a deeply-plicated species ; and the same explanation will probably apply to other 

 cases. 



