OE SOUTHERN INDIA. 67 



Dogger, the Ceromycu are almost characteristic, but some species also occur in 

 cretaceous deposits. 



23. Gresslya, Agassiz, 184^2, (Etud. crit., p. 202). Shell oval, generally 

 moderately compressed, beaks rather anterior, incurved, right valve always larger 

 than the left, the former internally behind the beak with a prominent longitudinal 

 rib, to which a cartilage was probably attached, fixing the upper margin of the left 

 valve to it. 



The surface of the shell is concentrically striated, and its structure is more solid 

 than in Ceromya, Terquem and others consider Gresslya to be identical with the 

 former genas, but even setting aside the marked difference in the form of the shell, 

 there seems to me a great difference in the hinge, which in Gresslya is more allied 

 to that of JEdmondia^ but in this we have according to Koninck an internal promi- 

 nent ridge in both valves, which also are perfectly equal ; the generic distinctions 

 appear to be, therefore, well founded. 



24. Allorisma^ King, 1844, (Perm. foss. of England, p. 196 j. Shell oval, 

 equivalve, inequilateral, very thin, hinge perfectly edentulous ; only an external 

 ligament is said to be present, attached to slightly thickened long fulcra; 

 anterior muscular impression near the ant ero -ventral margin ; pallial sinus small, 

 but distinct. This genus must most probably be reserved for a number of palaeozoic 

 species, unless some described under the next name are referable to it. 



25. Ilyacites^ Schloth., 1820. Shell oblong, thin, usually punctated, sub- 

 equivalve, inequilateral, posteriorly slightly gaping; hinge edentulous, indented, 

 posterior margin internally thickened, probably for the support of a cartilage ; 

 pallial sinus deep ; muscular impressions large. This genus is closely allied to 

 Allorisma^hMiiYiQTQ is no distinct trace of an external ligament; its form also 

 very much recalls Homomya, but in this strong nymphse are present externally, 

 while in Myacites we have them internally. (See Giebel in Abhandlungen 

 des Nat. Ver. fiir Sachsen und Thiiringen, vol. i, p. 103, pi. iii, fig. 8). It is not 

 satisfactorily known in palaeozoic strata, but it is abundant in the Trias and Jura, 

 and some species appear to be found also in cretaceous strata. Morris and Lycett 

 (Moll. Great Oolite, p. Ill,) identify Schlotheim's Myacites, (which was first 

 introduced for the typical triassic species) with Pleiiromya, Arcomya, Homomya, 

 and several other new genera proposed by Agassiz, but this is evidently an 

 inadmissible generalisation of characters, though it cannot be doubted that the 

 correct generic determination of our palseo-and meso-zoic Pelecypoda of this family 

 will long remain unsettled, and the examination of good materials will certainly 

 suggest many important alterations. 



I think it probable that Sowerby's genus Fachymya which is based upon a 

 cretaceous species, is merely a slightly different form of Myacites ; as regards form, 

 at least, both are undistinguishable, except that the former is more inflated poste- 

 riorly than anteriorly; no hinge teeth are known to exist. Pictet (Pal. Suisse, 

 iv. ser., p. 159,) places Fachymya in the Bonacidje, for which I cannot see 

 the slightest reason, unless the hinge has been examined which it is not stated to 

 have been. 



