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 Orthis pecten (= Anomia pecten, Linneus). 
Typifying Ortis with the shell last noticed, the genus becomes restricted to such 
species as O. arachnoidea, Phill., O. crenistria, Phill., O. eximia, Kichwald, and 
O. applanata, Salter, which have generally thin valves; the fissure closed with a 
deltidium ; and slender, isolated or projecting dental plates.2 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 nto 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 Ort/zs ; 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-striate (cluding species with the fissure generally open), and the 
ecto-striate (embracing those having the fissure closed with a deltidium). It is much 
to be regretted, that when this accomplished palzontologist 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 
sufficient 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 Gonambomtes plana, Pander, for several species of the group Lecto-striate ; 
while I have been induced to station the remainder in Ortdis, and to institute the 
genus Schizophoria for the group Arcuato-striate, adding to it certain aberrant species. 
1 There are doubtless many species of Orthis, even restricted as this genus is in the text, at present 
distributed in Leptena, Strophomena, &c. ; but without a knowledge of their internal parts, they cannot with 
any safety be instanced as examples under the present head. 
2 Tam 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, pl. xii 
and xiii; ‘ Bulletin de la Societe Géologique de France,’ 2™° série, 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. 1 agree with Mr. Sharpe as regards most of the characters with which he has invested the 
Anomia pecten of Linneus, and the distinctiveness he has pointed out between it and my present Stropho- 
menas (his Leptenas); 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, 
Lepteenas. 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. 
O 
