86 MOLLUSCA FROM THE CRAG. 
Diameter, *ths of an inch. 
Locality. Cor. Crag, Sutton, Ramsholt, Sudbourn, Gedgrave. 
Red Crag, Sutton, Bawdsey. 
Recent, Mediterranean, Britain, and Scandinavian Seas. 
Specimens of this species may be plentifully obtained in the lower or Coralline 
Crag Deposit, especially at Gedgrave, where it appears to have been abundant, and 
the numerous prominent and interlocking teeth being favorable for the preservation 
of the valves in their natural position, they are frequently found united. 
Considerable variation may be observed in comparing numerous individuals, some 
of which closely approach what is called a species in the recent state under the name 
of nitida, these differences consist principally in the proportionate dimensions, the 
posterior side occasionally projecting more in some specimens than in others, and in 
a greater solidity of shell. Faint traces of radiating strize ornament the exterior, they 
are most visible near the margin, these lines are also visible within; the number of 
crenulations slightly vary, being smaller and closer of course in the younger shell, and 
in this state the contour is generally more rounded, the posterior side in particular 
being less truncate. 
A species somewhat closely allied to this is found in the upper part of the Older 
Tertiary Formations at Hordwell, and in the Isle of Wight, but it is probably distinct. 
It is thinner, the hinge line narrower and more regular, with fewer teeth, while in 
NV. nucleus it is much broader as it approaches the anterior side, and the posterior 
margin is rather less rounded. 
A shell from the Upper Tertiaries of the United States strongly resembles this 
species, and may, when better known, prove to be an identity. 
5. NucuLa TRIGONULA, S. Wood. Tab. X, fig. 7, a—é. 
Nucuna tTriGonuLA. S. Wood. Ilust. Mag. Nat. Hist., vol. iv, New Series, p. 295, pl. 14, fig. 3. 
— — S. Wood. Catalogue, 1840. 
—_— —_ Morris. Cat. Brit. Foss., p. 95, 1843. 
Spec. Char. Testd minuta, trigonuld, levigatd, tumidd, margaritaced ; antice sub- 
angulata, postice abbreviatd, margine ventrali crenulato, 
Shell small, trigonular, smooth, tumid, nacreous, anterior side roundedly angular, 
posterior side very short, ventral margin crenulated. 
Longitudinal diameter, 2ths. Height, ¢ of an inch. 
Locality. Cor. Crag, Sutton. 
About a dozen specimens with a much more angular or trigonal form than 
is presented by the normal character of JV. nucleus, are in my cabinet from the Cor. 
Crag, which induced me, when publishing the Crag species of this family, to consider 
them as belonging to a distinct species, and I am still inclined to retain them as such, 
from a very marked difference in the outline; it is nevertheless possible they may be 
nothing more than specific variations, the result of some local condition ; they possess, 
