MECYNODON. 
33 
1. Mboynodon columbtnus, Whidborne, sp. PI. II, figs. 12 — 14, 
? 1834-40. Megalodus cabikatus, Goldfuss (pars). Petref. Germ., vol. ii, p. 183, 
pi. cxxxii, fig. 9 <7 (only). 
1889. Megalodon ? coltjmbinus, Whidborne. Geol. Mag., dec. 3, vol. vi, p. 79. 
Description. — Shell equivalve, very inequilateral, long, rather narrow, trans- 
verse, subquadrate or lozenge-shaped, convex, Umbones small, wide, depressed, 
proximate, recurved spirally forward, overhanging the anterior point of the hinge- 
margin. Hinge-margin slightly convex, about three-quarters the length of the 
shell. Anterior margin very convex, receding, Antero-inferior margin convex; 
postero-inferior margin concave ; inferior margin thus being sigraoidaL Posterior 
margin oblique, straight. Infero-posterior point much produced, acutely angular. 
Back divided by a very elevated, acute ridge, which rises at the back of the umbo, 
arches round in a slightly reversed sigraoidal curve to the postero-inferior point, 
and gradually becomes extremely lofty, so as to expand far beyond the rest of the 
shell, and in front of which the contour of the back is first concave, then nearly 
straight, and then bending in rapidly to the front margin. Surface, behind this 
line of greatest elevation, broadly concave, till it meets a much smaller ridge, 
which arches from the back of the umbo to the postero-superior point, and thus 
encloses a large, shallow, lanceolate escutcheon, bisected by the raised hinge- 
margin. No lunule. 
Surface covered, in front of the first ridge, with numerous (about thirty) 
distant, elevated, acute, transverse, rather irregular and unequal, divaricating 
threads, facing upwards, and separated by concave interspaces ; between the two 
ridges with similar, but much finer, more numerous, and more regular threads ; 
and on the area with a few indistinct bulges. Shell-structure massive. 
Hinge of the left valve with a deep, short groove, preceding a short, large, 
triangular, oblique central tooth, followed by a narrow gap containing an external 
ligament, beyond which is a long, large, narrow, horizontal posterior tooth. Pedal 
impression very deep and transverse. Muscular impressions rather large, shallow, 
and sublunate, Pallial line slightly notched in the centre of the posterior 
margin. 
Size. — Length 28 mm., breadth 21 mm., depth of both valves 20 mm. Another 
imperfect specimen is at least twice as large. 
Locality. — Chudleigh. There are three specimens in Mr. Vicary's Collection, ^/s-aA /v 
two of which are in beautifully perfect condition. 
BemarJcs. — It is rare indeed that both the externa) and internal characters of 
a Devonian shell are so well preserved as they are in these lovely fossils ; and it 
VOL. II. 5 
