margin slightly sigmoidal. Hinge-line straight, as long as the shell, its margin thickened on each valve, leaving in casts two long grooves. Umbones well developed, contiguous, but not touching, anterior, but not quite terminal, with a broad, very obtusely rounded diagonal ridge proceeding from each, to which the shell owes much of its exceedingly convex form. Byssal furrows shallow, most pronounced in the left valve; marginal notch not deeply excavated. Anterior muscular impressions quite anterior, infra-umbonal. Surface of the shell covered with concentric subimbricating lamellæ, crowded and striiform on the anterior end, but opening out and becoming lamellar on the diagonal ridge and posterior wing.
Obs. The shells comprised in this species very much resemble some Myalinæ; but I believe I am more justified in referring them to the present genus than to Myalina, of the striated hinge-plate of which I can find no trace ; neither has any definite evidence presented itself which would warrant me in placing them in either Avicula or Pterinea. The much more central position of the diagonal ridge, greater convexity of the shell, and the sigmoidal margin of the posterior end at once distinguish A. obesa from either Anthracoptera? or Myalina (Avicula) quadrata, Sow., A.? or M. (Avicula) modiolaris, Sow., A.? or M. (Modiola) carinata, Sow. [1], or Anthracoptera? Browniana, Salter (= Avicula tenua, Brown). I am not acquainted, and so cannot institute a comparison, with any of those Coal-measure fossils figured by Captain T. Brown in his 'Fossil Conchology'[2], many of which will doubtless fall into the genus Anthracoptera; but A. obesa appears to be quite distinct.
Loc. and Horizon. In a bed of hard micaceous sandstone, Drumsheugh, Water of Leith, at Dean Bridge, Edinburgh, Wardie-Shale division of the Cement-stone group; in altered shale underlying trap, Corstorphine Hill, near Edinburgh; in a band of limestone above the sandstone at Craigleith Quarry, near Edinburgh?
Genus Myalina, De Koninck.
Myalina, De Koninck, Anim. Foss. Terr. Carb. Belgique, p. 125.
Myalina crassa, Eleming (non G. & E. Sandberger).
Var. modioliformis, Brown.
Obs. In the 'Annals and Magazine of Natural History' for June 1875 I described a series of specimens of M. crassa, Elem., from the Fife Carboniferous-Limestone series and the Lower Carboniferous of this neighbourhood, and gave the synonymy of the species as then known to me. I remarked on the slighter make of the latter as compared with the Fife examples. I find that the Water-of-Leith shells from the Lower Carboniferous rocks were described
- ↑ In one place Mr. Salter referred these species to Myalina (Iron Ores Gt. Britain, 1861, pt. 3, p. 228), in another with doubt to Anthracomya (ibid. p. 230), and again in a third to Anthracoptera (Oldham Memoir, 1864, p. 64). He appears to have been in great doubt as to their true generic affinity.
- ↑ Plate 62.
