86 MOLLUSCA FROM THE CRAG. 



Diameter, fths of an inch. 



Locality. Cor. Crag, Button, 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 striae 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 

 N. 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 b. 



NTJCULA TRIGONULA. S. Wood. Illust. Mag. Nat. Hist., vol. iv, New Series, p. 295, pi. 14, fig. 3. 

 S. Wood. Catalogue, 1840. 

 Morris. Cat. Brit. Foss., p. 95, 1843. 



Spec. Char. Testa minutd, trigonuld, Icevigatd, tumidd, margaritaced ; antice sub- 

 angulatd, postice abbreviatd, margine ventrali crenulato. 



Shell small, trigonular, smooth, tumid, nacreous, anterior side roundedly angular, 

 posterior side very short, ventral margin crenulated. 



Longitudinal diameter, fths. Height, J 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 N. 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, 



