72 MOLLUSCA FROM THE CRAG. 
consists of about ten teeth, six upon the anterior or rounded side, these are placed 
nearly vertical, or forming an obtuse angle, and very prominent in the centre, the four 
teeth on the posterior or produced side are much inclined and nearly parallel with the 
hinge line, they are also obtusely angular, interlocking between those of the opposite 
valve, and by their prominence keeping the two portions united; hinge margin with a 
row of crenulations deeper, larger, and more visible within upon the posterior side : 
when the shell is perfect the exterior is finely decussated, the radiating striz not 
being more prominent than the lines of growth, but when the exterior coating is 
removed, which is generally the case more or less, the surface is strongly rayed or 
costated. In the young state the shell is less tumid than when full grown and less 
oblique. The muscular impressions are unequal in size and suborbicular, the anterior 
one or that upon the shorter side is situated close up to the hinge line; the one 
on the posterior or opposite side is larger and much nearer to the ventral margin. 
This shell in form and magnitude bears a resemblance to Pectunculus nanus, 
Deshayes, an Eocene fossil from the Paris Basin, figured and described in his ‘ Hist. 
des Coq. Foss. des Env. de Paris, vol. i, p. 226, t. 36, figs. 4, 5,6; but judging from the 
figure as well as from the description, there appears a different arrangement of the 
teeth or denticles, those of the French shell have the greater number upon the larger 
or produced side, amounting to as many as six, with only three or four upon the other 
or shorter side, and are less oblique, thus reversing the dental arrangement of the Crag 
shell; and the French fossil is said to be thin and fragile, while ours is thick and strong. 
The triangular fossette of the Crag shell is deep, and forms an angle less than 90°, 
and the margin of the shell is perfectly closed all round. 
NucIne.ia, S. Wood. 
Nucwua (sp.). Deshayes, 1829. 
Pievropon. S. Wood, 1840. 
Nucuuina. D'Orbigny, 1845, sec. Gray. 
Gen. Char. Shell equivalve, inequilateral, closed, ovate or subtrigonal; anterior 
side short, truncate; posterior produced, ovate or subangular; hinge line broad 
slightly curved, furnished with few teeth: one large lateral tooth on the posterior 
side. Ligament external. 
ANIMAL UNKNOWN. 
The diagnosis of this was drawn up from what may be considered as scarcely 
sufficient materials for the distinction of a group of animals, denominated a Genus, 
being founded upon but one species; it presents, however, such marked differ- 
ences in character from any genus hitherto established, that I was unable to find 
a position for my little shell when it was first described. The outward trigonal 
