104 PERMIAN FOSSILS. 



most of which are plano-convex, or concavo-convex medio-longitudinally (indifferently 

 as regards which valve) ; and more or less semicircular marginally ; and have strong 

 regular valves ; thick curving dental plates ; the point of the umbone occasionally 

 foraminated ; and the fissure more or less covered with a deltidium.* 



Several of the foregoing characters distinguish StropJiomenairora. Leptana (;=.Lepta- 

 gonia, M'Coy), a genus subsequently (1827) founded by Dalman on XhQ Produdaruffosa 

 of Hisinger, and improperly made the receptacle for shells belonging to the former, 

 both by Dalman himself, and many palaeontologists of the present day. Considering 

 Leptcena to be typified by the shell last named, this genus, although agreeing in some 

 respects with Strophomena, obviously differs from it in having an open fissure -^ the 

 valves geniculated in front, and transversely wrinkled ;3 and the umbone often furnished 

 with a foramen. Leptana clearly embraces such shells as L, analoga, Phill., L. semi- 

 ovalis, M'Coy, L. plicotis,^ M.' Coy, L. nodulosa, Phill., and L. multirugata, M'Coy. 



There are many fossil shells belonging to the present family, such as Produdus 

 comoides, J. Sow., Pledambonites ohlonga, Pander, Orthis ornata, Eichwald, and a few 

 others, which some may be disposed to regard as militating against the distinctiveness 

 of the last two genera'; but I am more inclined to consider certain of them to be 

 either simulating or merging forms, and the remainder as typical of undescribed genera. 



At the same time Dalman instituted Leptcena, he proposed the genus Orthis, 

 describing it, as is too often the case, from a number of very different shells, though 

 agreeing in some of their characters. The diagnosis of the genus is as follows : " Testa 

 insequivalvis, aequilatera ; valva minori subplana, majori subconvexa. Margo cardi- 

 nalis rectilineus, latus, foramine deltoidea sub nate valvse majoris. Valva major dentibus 

 duobus subcardinalibus internis, longitudinalibus, compressis."^ Dalman, Un placing 

 nine species in the genus, evidently entertained the idea that they had all an open 

 fissure, and were congeneric forms ; but what is the fact ? — they belong to four 

 distinct genera ; viz., OrtJds, Chonetes, Orthisina, and Sddzoploria ; and it is very 

 doubtful whether more than two or three species have an open fissure.^ It may be 



^ A species pointed out to me by Mr. T. Davidson, and named Strophomena latissima, Boucliard Chanteraux, 

 does not appear to have had either a deltidium or fissure ; — probably it possessed the former in the young state. 



- Mr. Sharpe, who objects to the separation I formerly proposed between Strophomena and Leptcena, 

 overlooks my urging, as one of the distinctive characters of the last genus, the open fissure seen in all its 

 species. (Vide Ann. and Mag. Nat. Hist., vol. xviii, p. 38 ; and Geol. Journ., vol. iv, p. 179.) 



^ Some species of Strophomena simulate Leptana in being transversely wrinkled. There is an approach 

 to the same character in the singular Strophomena described by De Verneuil under the name of Leptcena 

 Loveni. (Vide Bull. Geol. Soc. de Frauce, 2"° s^rie, t. v, pi. iv, fig. 5 a, b, c, d.) 



i Is the large valve oi Leptcena plicotis, the concave one ? (Vide Synopsis of the Silurian Fossils of Ireland, 

 pi. iii, fig. 5.) 



5 K. Vet. Acad. Handl. p. 26, 1827. 



•^ As a further reason for not attending to the characters given by Dalman, it may be stated that 1 the 

 so-called Orthises figured in Hisinger' s ' Lethaea Suecica,' are represented with an open fissure — even Orthis 

 pecten. 



