POSIDONOMYA. 
53 
Anterior side very broad and nearly straight. Inferior margin broad and convex. 
Posterior margin gently convex. Contour steep on the anterior side, very flat 
elsewhere. Posterior wing small, short, broad, triangular, flat, denned by a 
depression bounding the rear of the umbo. Surface marked with numerous 
minute, distant, irregular strige or growth-lines. Shell-structure thin. 
Size of single valve. — Length 20 mm., breadth 26 mm., depth 5 mm. 
Locality. — Lummaton. A single specimen is in my Collection. 
Bemarhs. — The evidence afforded by this specimen is far from satisfactory. 
The shell appears to be distinguished by its direct ovoid form, which is broader in 
the lower part, its flatness, its broad low umbo which arches forward and is bounded 
on each side by steep lines, and its small triangular hind wing, which is right- 
angled and isosceles. Thus it seems to be distinctive, and is certainly unlike any 
of the species which accompany it. 
Affinities. — It appears to be very similar in general shape to Mytilarca 
{Plethomytilus) oviformis, Hall,^ but to be a shorter and flatter shell, with a less 
incurved umbo. 
II. Sub-family. — Aviculin^, StoliczJca, 1871. 
1. Genus. — Posidonomya, Bronn, 1837. 
This genus extends from the Silurian to the Trias, but is especially charac- 
teristic of the Culm, and appears rare in the Devonian. It is, therefore, 
interesting to find it represented in the present beds. The species described 
below appears to belong to Posidonomya as restricted by de Koninck in 1887, 
who separa.ted many of its forms under the name of Posidoniella. 
1. Posidonomya oblonga, Trenlcner, sp. PL IV, fig. 8. 
1867. Aticula oblonga, Trenlcner. Palaont. Novit., pt. 1, p. 22, pi. iii, fig. 48. 
1889. PosiDONOMXA LATERALIS, Wlddhorne. Geol. Mag., dec. 3, vol. vi, p. 78. 
Description. — Shell rather small, direct, semioval, much higher than long, 
♦ slightly convex. Umbo small, low, sharp, slightly twisted anteriorly, apparently 
not elevated above the hinge-line. Hinge-line apparently straight, and equal 
to the length of shell. Anterior and posterior margins very broad, probably 
slightly sinuous, and meeting the hinge-margin at high angles. Inferior margin 
1 1884, Hall, 'Pal. N. Y.,' vol. v, pt. 1, sect. 1, p. 255, pi. xxxi, figs. 1—8 ; and pi. Ixxxvii, fig. 8. 
