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 pomt of the umbone occasionally 
foraminated; and the fissure more or less covered with a deltidium.* 
Several of the foregoing characters distinguish Strophomena from Leptena (= Lepta- 
gona, M‘Coy), a genus subsequently (1827) founded by Dalman on the Producta rugosa 
of Hisinger, and improperly made the receptacle for shells belonging to the former, 
both by Dalman himself, and many paleontologists of the present day. Considering 
Leptena 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. Leptena clearly embraces such shells as Z. 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 Productus 
comoides, J. Sow., Plectambonites oblonga, Pander, Orthis ornata, Kichwald, and a few 
others, which some may be disposed to regard as militating against the distinctiveness 
of the last two genera; but Iam 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 Zeptena, he proposed the genus Orihis, 
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 
ineequivalvis, zequilatera ; valva minori subplana, majori subconvexa. Margo cardi- 
nalis rectilmeus, latus, foramime deltoidea sub nate valvee majoris. Valva major dentibus 
duobus subcardinalibus internis, longitudinalibus, compressis.”” 
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., Orthis, Chonetes, Orthisina, and Schizophoria; and it is very 
doubtful whether more than two or three species have an open fissure.’ It may be 
Dalman, in placing 
' A species pointed out to me by Mr. T. Davidson, and named Strophomena latissima, Bouchard Chanteraux, 
does not appear to have had either a deltidium or fissure ;—probably it possessed the former in the young state. 
2 Mr. Sharpe, who objects to the separation I formerly proposed between Strophomena and Leptena, 
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.) 
3 Some species of Strophomena simulate Leptena in being transversely wrinkled. There is an approach 
to the same character in the singular Strophomena described by De Verneuil under the name of Leptena 
Loveni. (Vide Bull. Géol. Soc. de France, 2™ série, t. v, pl. iv, fig. 5a, 6, ¢, d.) 
4 Ts the large valve of Leptena plicotis, the concave one? (Vide Synopsis of the Silurian Fossils of Ireland, 
pl. iti, fig. 5.) 
5 K. Vet. Acad. Handl. p. 26, 1827. 
6 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 ‘ Lethzea Suecica,’ are represented with an open fissure—even Orthis 
pecten. 
SS 
