MYALINA VERNEUILII. 115 



has enabled me to ascertain these details. Specimens show in reverse all the 

 characters of the interior : the thickened hinge-plate, the hollow between the 

 internal umbo and the anterior edge of the shell for the rostral plate, the remote 

 entire pallial line, and the position of the insertion of the posterior adductor 

 muscle. 



The type of M'Coy's Mytilus comptus has unfortunately disappeared, but I 

 think it very probable that it was one of these intermediate forms. Mr. Neilson 

 possesses a specimen, having the same outline as that given in M'Coy's figure, 

 which was obtained with typical M. Flemingi from the above-mentioned locality, 

 and a similar series of varying forms occurs at Pule Hill. 



Mtalina Verneuilii, sp., M'Coy, 1844. Plate III, fig. 6; Plate IV, figs. 3—8. 



Avicula Vebnettilii, M'Coy, 1844. Synops. Charact. Carbonif. Foss. Ireland, 



p. 85, pi. xiii, fig. 19. 

 — — d'Orbigny, 1850. Prodrome Paleont., p. 136. 



Mtalina Veeneuilii, Armstrong and Young, 1871. Carbonif. Fossils West of 



Scotland, p. 54. 

 Avicula Veeneuilii, Etheridge, 1885. Brit. Foss , vol. i, Palseoz., p. 286. 



Specific Characters. — Shell transversely subtrigonal, moderately flat. The 

 anterior surface makes an obtuse angle with the rest of the shell, and projects 

 above as a very small lobe, immediately below which the surface becomes slightly 

 concave, while below it is convex from side to side, and from above downwards, 

 narrowing to an edge where it joins the inferior border, which is short and 

 obtusely rounded, the anterior limb of the curve being blunter and less regular 

 than the posterior. 



The posterior border is obliquely truncate from above downwards and back- 

 wards, slightly emarginate above, where it makes an obtuse angle with the hinge- 

 line. The hinge-line is straight, and extends the whole length of the upper 

 border, and forms an acute angle in front with the anterior edge of the shell. 



The umbones are swollen, small, narrow, and terminal. Proceeding down- 

 wards and backwards from the umbones, and continuous with them, is a narrow 

 ridge, which forms the angle between the anterior and lateral surfaces. This is 

 gently rounded on either side, and becomes less and less conspicuous as it 

 approaches the lower part of the shell, and is lost at the junction of the front and 

 lower borders. The lateral surfaces of the shell are compressed and flattened. 



The Interior. — In casts the umbo is not terminal, but is well marked off from 

 an anterior lobe by a sulcus, which contained the process in which was the 

 anterior adductor muscle-scar. The posterior adductor scar is large, circular, and 



