184 MOLLUSCA FROM THE CRAG. 



Shell roundcdly trigonal, slightly inequilateral, often somewhat oblique, and 

 deformed ; posterior side the broader or higher ; dorsal margin rather rounded ; 

 externally sulcated, or covered with numerous rounded ridges ; margin smooth. 



Diameter, Jths of an inch. 



Localities. Red Crag, Sutton, Alderton, Bawdsey, AValton-on-the-Naze. 

 Mam. Crag, Bridlington, Branicrton, Thorpe. 



Clyde Beds and Uddevalla. Recent, Britain, Scamlhiavia. 



This species is not by any means rare in the Red Crag, but the shell found there 

 is generally that variety which has the lesser number of ridges, although amongst 

 my specimens there is a variation from 30 to nearly 50. In the newer deposits of 

 Bramerton and Bridlington the specimens are often nearly smooth, or have a greater 

 number, and finer stria;, while the shells also are often somewhat deformed, with a 

 greater comparative length, and a more quadrate kind of outline, and the apices are a 

 little eroded. 



What may be called the normal form (fig. 8 a) is subtrigonal, with a diameter as 

 great from the umbo to the ventral margin as from the anterior to the posterior side, 

 the umbo curving a little towards the anterior; the shell is generally somewhat 

 compressed, and the ridges of the exterior are rounded and smooth, and about as 

 broad as the spaces between them ; the liinge is not very broad, and there is a 

 somewhat prominent anterior lateral tooth at the extremity of a large and elongated 

 lunule, and the dorsal margin is generally more or less rounded. The Red Crag 

 furnishes specimens very variable in outline and somewhat deformed^ but they are 

 always regularly ridged and sulcated. 



The recent specimens do not often exceed half an inch in diameter, and have been 

 obtained in ten as well as in eighty fathoms. 



11. AsT.\UTE CREimiLiR.VTA, S. Wood. Tab. XVI, fig. 2 a — b. 



Spec. Char. Testa transversa, ovato-trigond, inmquilaterali, tumidimculd, crassd; postice 

 lortffiore, sul)truncatd ; striatd, striis crebris obtusis confertis, lunidd elongaid, excavatd, 

 niargine cremdato. 



Shell transverse, ovately triangular, or wedge shaped, inequilateral, slitihtly tumid, 

 thick ; posterior side the longer, and truncated, externally striated or ridged ; stria; 

 numerous, close, rounded, and obtuse ; lunule elongated, deep ; margin crenulated. 



Length, Jtlis of an inch ; height, f,ths ditto. 



Localities. Red Crag, Sutton, Walton-on-the-Naze. 



Recent, Arctic Seas.. 



A few specimens only of what appears to be a distinct species have been obtained 

 by myself from the Red Crag. They correspond very closely, and in such a variable 

 genus as this, may perhajjs be considered as identical with a living species in the 

 Cabinets of Messrs. Morris and Lowry, and said by those gentlemen to have been 



