ANIMALS. 105 



asked, what is to be done in such a case ? My answer is — abandon the diagnosis, as 

 it has evidently been drawn upunder an erroneous impresssion, and attend solely to 

 the characters of the species first described under the head of the genus, — the one 

 named OrtJds peden {= Anomia pecten, Linnaeus). 



Typifying Orthis with the shell last noticed, the genus becomes restricted to such 

 species as 0. arachnoidea, Phill., 0. crenistria, PhilL, 0. eximia, Eichwald, and 

 0. applanata, Salter,^ which have generally thin valves ; the fissure closed with a 

 deltidium ; and slender, isolated or projecting dental plates.^ The thinness of its 

 valves, the character of its dental plates, and the absence of a foramen, distinguish 

 Orthis from Strophomena, into which on the one hand it undoubtedly passes, while 

 on the other, it graduates into M. A. d'Orbigny's Orthisina, 



We owe to M. de Verneuil the fullest details on the large assemblage of fossil species 

 usually placed in the genus Orthis ; and it is entirely to his researches that we are 

 indebted for a knowledge of the two great groups to which these species are referable 

 — the Arcuato-striatcB (including species with the fissure generally open), and the 

 Recto-striatcB (embracing those having the fissure closed with a deltidium). It is much 

 to be regretted, that when this accomplished palaeontologist was describing those fossils 

 in the great work on ' Russia and the Ural Mountains,' he did not elevate these groups 

 to the rank of genera ; for it is quite evident that each one possesses characters of 

 sujQ&cient weight and importance to constitute a generic diagnosis. As it is, others, 

 availing themselves of the ample materials thus prepared for them, have been led to 

 institute genera, with certain modifications, for the groups first pointed out and defined 

 by M. de Verneuil. Thus, M. A. d'Orbigny has already proposed the genus Orthisina, 

 typified by Gonambonites plana, Pander, for several species of the group Becto-striata ; 

 while I have been induced to station the remainder in Orthis, and to institute the 

 genus Schizophoria for the group Arcuato-striata, adding to it certain aberrant species. 



1 There are doubtless many species oi Orthis, even restricted as this genus is in the text, at present 

 distributed in Leptcena, Strophomena, &c. ; but without a knowledge of their internal parts, they cannot with 

 any safety be instanced as examples under the present head. 



2 I am in a great measure indebted to Mr. D. Sharpe, M. J. W. Salter, and Mr. T. Davidson (vide 

 ' Quarterly Journal of the Geological Society of London,' vol. iv, part i, pp. 178-80; 'Memoirs of the 

 Geological Survey of Great Britain,' vol. ii, part i, pp. 371-81 ; 'London Geological Journal,' vol. i, pi. xii 

 and xiii ; 'Bulletin de la Societe G^ologique de France,' 2"° serie, t. v, p. 309, &c.) for information qualify- 

 ing the conclusion I have arrived at respecting the distinctiveness of the shells here placed in Orthis and 

 Strophomena. I agree with Mr. Sharpe as regards most of the characters with which he has invested the 

 Anomia pecten of Linnaeus, and the distinctiveness he has pointed out between it and my present Stropho- 

 menas (his Leptsenas) ; we differ, however, as to the generic names which these two groups ought re- 

 spectively to bear, — my Orthises being termed Strophomenas by Mr. Sharpe, and my Strophomenas, 

 Leptsenas. It would have afforded me much pleasure to have agreed with Mr. Sharpe in this respect ; but 

 my differing from him is entirely through being firmly persuaded that the rule I have throughout adhered 

 to in this work, and elsewhere noticed, is the best calculated to divest generic nomenclature of its present 

 confusion. 







