62 
DEVONIAN FAUNA. 
From the type specimen of Mytilus Damnoniensis, Phillips, this species differs 
by its very narrow and concave hind wing. 
Mijtilus dimidiatus, Goldfuss, sp./ and Ferd. Romer/ is less ovoid inferiorly, 
and less produced in the postero-superior region. 
Mytilus intumescens, F. A. Romer,^ is also very similar, but as seen in a 
German example in the British Museum is a broader and much deeper shell, with 
a longer umbo. Its back appears to be more evenly convex, and the greatest 
depth is much nearer the inferior margin. 
Myalina ornata, F. A. Romer,* and Gardiomorpha mytiloides, F. A. Romer,^ 
are both more decidedly cuneiform shells. 
Mytilarca nitida, Billings," is exceedingly similar in general form. Its umbo 
seems larger and more prominent, its anterior side is concave near the beak, and 
there are several other features which show it to be distinct. 
Mytilus pygmseus, Goldfuss,' of the Mountain-limestone, is a smaller, broader, 
and much less angulated shell. 
Mytilarca gibhosa, Hall,^ is also extremely like. It chiefly differs in having a 
more rounded and shorter umbo, and a shorter and less oblique hinge-line. 
Myalina solida, Maurer,^ is rather longer, and not so deep above. 
3. Genus. — Plethomttiltjs, Hall, 1883. 
This genus is said to differ from Mytilarca in its erect form, its transverse 
and shorter hinge-line, the absence of teeth, and its non-truncate anterior side. 
1. Plethomytilus ketoetus, n. sp. PI. V, fig. 1. 
Description. — Left valve rather small, short, broad, ovoid, direct, flatfish. 
Umbo small, acute, elevated, proximate, compressed, arching rather forward, and 
situate at the anterior extremity. Hinge-margin apparently short and straight. 
1 1834i-40, Goldfuss, ' Petref. Germ.,' vol. ii, p. 284, pi. clx, figs. 16 a, b. 
2 1876, E. Eomer, ' Lethffia Pal.,' pi. xxix, figs. 3 a, b. 
3 1850, F. A. Eomer, ' Beitr.,' pt. 1, p. 32, pi. v, fig. 1 ; and 1891, Freeh, ' Abhandl. Geol. 
Specialk. Preuss,' Band ix, pt. 3, p. 156, pi. xvii, figs. 16 — 16 b. 
* 1860, ibid., pt. 4, p. 162, pi. xxv, figs. 12 a, b. 
Ibid., p. 163, pi. xxv, figs. 11a, 6. 
6 1874, Billings, ' Palfeozoic Fossils,' vol. ii, pt. 1, p. 53, figs. 22, 23. 
7 1834-40, Goldfuss, ' Petref. Germ.,' vol. ii, p. 168, pi. cxxviii, figs. 6 a, h. 
8 1885, Hall, ' Pal. N. Y.,' vol. v, pt. 1, sect. 1, p. 262, pi. xxxiii, fig. 20 ; pi. Ixxxvii, fig. 7. 
9 1886, Maurer, 'Fauna Eechtsrhein. TJnterdevon.,' p. 13; and 1891, Freeh, 'Abhandl. Geol. 
Specialk. Preuss,' Band ix, pt. 3, p. 143, fig, 15. 
