114 CARBONIFEROUS LAMELLIBRANCHIATA. 



acute, terminal, curved towards the anterior side; posterior margin convex." 

 This description lays stress on the characteristic form. 



In his observations M'Coy says, " This species, like Dressina polymorpha, has 

 the truncated anterior face and ridge flexuous or sigmoidal." The external 

 resemblance between this species and Dreissensia polymorpha is so close that, were 

 it not for the characters of the hinge, undoubtedly the two shells would have been 

 referred, if not to the same species, at least to the same genus ; and I can see no 

 external character to separate the fossil and recent forms. It is curious that two 

 shells, living at such widely separated periods of time, should resemble each 

 other so closely in external characters, and should possess such dissimilar hinges, 

 though both possess a special shell-process for the insertion of the anterior adductor 

 muscle. Was this form of Myalina the ancestor of the recent Dreissensia, in 

 which the striated hinge-plate has been gradually atrophied and finally lost ? 



There can be little doubt that the Mytilus Beaumonti of de Verneuil is identical 

 with M'Coy's shell ; and although de Verneuil was aware of the similarity between 

 them, owing probably to the meagre description given by M'Coy, he did not feel 

 certain of the identity of the Russian and Irish specimens. 



De Koninck expressed doubt as to the reference of this shell to Mytilus, and in 

 the explanation of his pi. xxiv gave the name Mytilarca, Hall, to it, but 

 without any generic description. He thought that the cardinal border was not 

 striated, and so differed from Myalina, and that the shell might belong to 

 Anthracoptera, Salter. This is not the case : for that genus, which is synony- 

 mous with Naiadites, Dawson (' Monogr. Carbonicola, &c.,' 1895, p. 129), I have 

 shown, possesses a striated hinge-plate, but no shelf for the anterior muscle-scar, 

 and the anterior part of the shell is lobular and not flattened into a surface, 

 and the umbones are not terminal. De Koninck's reference to Mytilarca on the 

 assumption of the absence of a striated hinge-plate was also equally erroneous, 

 as this genus also possesses this character, and both cardinal and posterior 

 teeth. De Koninck thought (1885) that the striated hinge-plate was absent from 

 a specimen partially stripped of its shell, and figured in his pi. xxix, fig. 22; but 

 this specimen, from its shape and outline, seems to belong to quite another species. 

 M. Flemingi appears to be somewhat variable in shape and thickness, and some 

 specimens have indication of an anterior lobe, the upper and median part of 

 the anterior surface being somewhat produced, and much less flattened than 

 in M. Redesdalensis. The Belgian specimens are somewhat finer than those I 

 have yet seen from British rocks. 



Fig. 10, PI. IV, is a fairly perfect specimen from a bed of calcareous sand- 

 stone from the Garngad Road, Glasgow. At the time when the plates were 

 drawn I had not seen the interior of the hinge. The discovery of a bed of marine 

 fossils in the Millstone-grit of Pule Hill, Marsden, by Messrs. Barnes and Holroyd, 



