MYTILARCA. 
51 
2. Genus. — Mytilaeca, Hall, 1870. 
This genus contains mytiliform shells with sharp terminal umbones, straight 
hinge-lines, and straight perpendicular anterior margins. The hinge has two or 
three strong cardinal teeth, and is obliquely striated behind. It belongs to the 
Devonian age. 
Whether the shell next to be described belongs to this genus must remain 
doubtful for the present, as its hinge is unseen. I have provisionally placed it 
here on account of its great similarity in general shape to some of the other 
species referred to the same genus by different authors. It may, however, prove 
to be a Mytilus or Myalina. 
1. Mttilaroa (?) RoBERTSii, WMdbome, sp. PI. Ill, figs. 7, 7 a. 
1889. Mttilus Eobertsii, Whidhorne. Geol. Mag,, dec. 3, vol. vi, p. 79. 
Description. — Shell almond-shaped, rather large, very high, convex. Umbo 
prominent, highly incurved, proximate, anteriorly truncated, slightly turned 
forward. Hinge-margin short, oblique, slightly curved. Anterior margin very 
broad, straight, and perpendicular, having a long linear slit for the byssus under 
the umbo. Posterior margin slightly convex. Inferior margin probably roundly 
convex. Hind wing small, concave, and very narrow. Line of greatest elevation 
running from the umbo parallel with and close to the front margin ; the contour 
of the shell being gently convex over and behind it, and perpendicular and flat 
in front of it. Surface and hinge unseen. 
Size of double shell. — Length 20 mm., breadth about 35 mm., depth 16 mm. 
Locality. — Wolborough. There is a specimen in Mr. Vicary's Collection, and 
another in the Museum of Practical Geology. 
Remarlcs. — This shell is distinguished among other features by its straight, 
flat, anterior side, its narrow, concave hind wing, and its deep, incurved umbones. 
Its greatest depth is very high up and near the umbo. 
Affinities. — It bears a general external resemblance to Myalina crassa, Sand- 
berger.^ Mr. Roberts, however, does not think that it ought to be placed with 
that species. There is a little ambiguity in Sandberger's drawing which makes the 
exact shape of his shell uncertain ; but it appears to have been much more evenly 
convex, and more acute and flattened above, and to have had a sharper umbo than 
the present species. 
1 1853, Sandberger ' Verst. Khein. Nassau,' p. 281, pi. xxix, figs. 12, 12 a. 
