54 
DEVONIAN FAUNA. 
short and roundly convex. Contour of back gently convex horizontally, almost 
straight perpendicularly. Anterior and posterior wings broad, triangular, flat, 
defined by indistinct depressions on each side of the umbo, which meet the 
margins about halfway down. Surface covered by seven or eight very much 
elevated, sharp, triangular, foliaceous ridges, separated by broad, concave inter- 
spaces, and becoming much larger and more distant as they approach the margins. 
8ize. — Length about 14 mm., breadth about 21 mm. 
Locality. — There is a single specimen from Wolborough in the Torquay 
Museum, and two from the same locality in Mr. Vicary's Collection. 
Remarks. — These shells are in poor condition, and their surface is decayed. 
They are distinguished by their few and very elevated ridges, their very central 
umbones, their slight obliquity, and, as far as can be judged, their wide straight 
hinge-lines and angular wings. They appear so exactly to correspond with 
Avicula oblonga of Trenkner that I have no hesitation in referring them to 
that species. 
Affinities. — Phillips^ figures two young specimens of Posidonomya lateralis, 
Sowerby,^ from the Culm, which resemble ours in the strength and fewness of the 
ridges ; but they seem to be more oblique, to be more rounded superiorly, and to 
have less central umbones. Sowerby's species as originally described is clearly 
distinct, as these differences are more clearly seen in it, and the ridges are, 
moreover, much more numerous. 
Posidonomya (.'') elongata, F. A. Komer,^ seems extremely similar as far as can 
be judged from its very slight description, but it differs in being very much smaller, 
higher, and more oblique, and therefore is not, I think, specifically identical. 
F. A. Romer* regards P. lateralis, Sow., as a variety of P. Becheri, Bronn,^ 
and the figure which he gives of it differs from our shells in being much wider and 
having much finer ornament. 
Tryon's^ figure of P. Becheri is rather similar in shape, but is more oblique 
and has finer ridges. 
Trenkner compares his A. oblonga with Avicula {Leiopteria) Benedeniana, 
de Koninck,^ but that shell differs from it in many particulars, generic as well 
as specific. 
The ridges in Avicula vasta, Trenkner,^ are very much weaker. 
1 1841, Phillips, ' Pal. Foss.,' p. 45, pi. xx, figs. 74 h. 
2 1840, Sowerby, ' Geol. Trans.,' ser. 2, vol. v, pt. 3, pi. lii, fig. 1. 
3 1850, F. A. Eomer, ' Beitr.,' pt. 1, p. 1.3, pi. iii, fig. 2. 
4 1843, F. A. Eomer, ' Verst. Harzgeb.,' p. 21, pi. vi, fig. Id. 
5 1828, Bronn, ' Jahrb.,' p. 262, pi. ii, figs. 1—4. 
« 1884, TryoD, ' Struct. Conch.,' vol. iii, p. 274, pi. exxx, fig. 46. 
7 1885, de Koninck, 'Ann. Mus. Eoy. H. N. Belg.,' vol. ix, pt. 5, p. 194, pi. xxx, fig. 19. 
8 1868, Trenkner, ' Palaont. Novitat.,' pt. 2, p. 13, pi. x, fig. 11. 
