46 
DEVONIAN PAUNA. 
the sliell), straight except that it arches slightly downwards at the umbo. 
Anterior margin very broad, slightly sigmoidal, being gently concave in the upper 
part and becoming convex below. Inferior margin narrow, regularly ovate. 
Posterior margin apparently gently convex, and meeting the hinge-margin at a 
high angle. Lunule lancet-shaped, indistinct. Ligament external (?). Deepest 
part of shell about one-third from the umbo. Line of greatest elevation running 
near the anterior side from the umbo to the inferior margin, over which the surface 
is strongly convex, and from which it slopes very rapidly to the margin in front, 
and very gently and flatly to the margins behind. Hind wing large, indefinite, 
and slightly concave. Surface smooth except for a few indistinct growth-lines. 
Shell-structure thin on the back, massive near the hinge-margin, consisting of 
two layers, of which the inner is dense and papyraceous, and the outer thick and 
crystalline. 
Size. — Length 27 mm., breadth 51 mm., depth of both valves 16 mm. Another 
specimen is 106 mm. in breadth. 
Locality. — Wolborough. There are two specimens in Mr. Vicary's Collection, 
and a third (indistinct) example in the Museum of Practical Geology. 
Remarks. — This species is the largest bivalve that I have seen from the present 
localities. Its hinge and interior are unknown, but its external characters are 
well displayed, and so much resemble those of the numerous Carboniferous species 
of Myalina given by de Koninck that I have little hesitation in referring it to 
that genus. These all, however, differ from it specifically, and in most of them 
the front wing is rather more definite. 
The species is distinguished by its flattish subovate form, being nearly 
straight above and ovate below, gently flattened rearward and steep in front, and 
having a small undefined umbo at the anterior extremity. 
Affinities. — Mr. Roberts suggested to me that it might be the same as 
Mytilus Damfioniensis, Phillips, but an examination of Phillips's type of that 
species leaves no doubt that that view cannot be sustained. The umbo of that 
species is much more erect, the contour of its back is much more evenly convex, 
and its hind wing is decidedly more definite. 
The larger of Mr. Vicary's specimens has been labelled (apparently by 
Salter) " Pterinea plana, Goldfuss, ' Petref.,' pi. cix, fig. 4." This identification 
is due to a rather odd mistake. The specimen is curiously like the Mesozoic 
Inoceramiis nohilis, to which the above reference belongs, but is totally unlike 
Pt. plana, loc. cit., pi. cxix, fig. 4. The smaller and more perfect shell, however, 
shows that the likeness to /. nohilis is entirely superficial. 
Mytilus Ihergensis, F. A. Romer,^ has the anterior side much steeper, the 
hinge-line shorter, and the posterior margin more arched. 
1 1860, F. A. Eomer, 'Beitr.,' pt. 4, p. 162, pi. xsv, figa. 10a, b. 
