32 
DEVONIAN FAUNA. 
Goldfuss, Bronn, and others give B. abhreviatus as a synonym for M. eucullatus, 
but if this be so the former name clearly carries the priority. Schlotheim's figures 
have every appearance of being the same species as those of Goldfuss, &c. The 
only differences I note are that in the former the keel which encloses the escutcheon 
meets halfway down the posterior side instead of at its base, so that the escutcheon 
is shorter; and also that the base is only gently convex, as in Goldfuss's figure of 
the young shell, and unlike the deeply convex base of his full-grown example. 
Sowerby's and Phillips's figures differ more considerably. They represent a 
very high variety with a smaller beak, from which the front margin runs almost 
straight down ; but the arrangement of the hinge is similar, and there appears to 
be no sufficient reason for separating them. 
Keferstein regards M. elongatus, ¥. A. Romer, as an old and worn example of 
the same species ; and this view is certainly supported by the German specimens 
in the British Museum. 
M. Adolfi, Clarke/ is very similar. Clarke distinguishes it by its less recurved 
and swollen umbo, its less concave lunule, its straighter front margin, and its 
broader and more flattened shell. It may, however, be questioned if it be more 
than a variety ; at all events, in these points it approaches the British form. 
III. Family. — AsTARTiDiE, Gray, 1840. 
1. Oenus. — Mecynodon, Keferstein, 1857. 
Shell long, concentrically striated or smooth, with a diagonal keel extending 
from the beak to the posterior margin. Ligament external, short. Hinge with a 
long elevated cardinal tooth parallel to the hinge-margin, with a pit posterior to it 
in the right valve, and anterior to it in the left ; and with a long and stout posterior 
lateral tooth. Anterior muscular impression deep, with a smaller but deeper 
impression of the foot-retractor behind it ; posterior muscular impression near the 
middle of the posterior margin. 
This definition is almost in the words of Keferstein, Zittel, and Tryon. I am 
not, however, able to understand all its points by a comparison either of our 
specimens or of Goldfuss's figures, to which Keferstein refers. 
In external shape it has considerable likeness to the genus Goniophora, 
Phillips, as G. Hauchicornei, Beushausen,^ indicates ; but that genus has no 
lateral teeth. 
1 1884, Clarke, 'Neues Jahrb. f. Min.,' Beil.-Band iii, p. 376, pi. vi, figs. 16, 17. 
2 1884, Beushausen, ' Abliandl. Geol. Specialk. Preuss.,' Band vi, pt. 1, p. 113, pi. hi, fig. 1. 
