174 MOLLUSCA FROM THE CRAG. 



AsTAKTE TRIANGULARIS. Alder. Catal. North, and Durh., p. 87, 1848. 



— — Jeffreys. Ann. and Mag. Nat. Hist., vol. viii, p. 166. 



— — Forb. and Hani. Hist. Brit. Moll., vol. i, p. 467, pi. xxx, figs. 4, 5, 



1848. 



— LAEVIGATA? PM. En. Moll. Sic, vol. ii, p. 30, 1844. 

 Crassina TRIANGULARIS. Gray. Ann. of Philos., 1825. 



MINUTISSIMA. Id. 



— MINIMA? Smith. Mem. Wern. Nat. Hist. Soc, vol. viii, p. 45, 1838. 



Spec. Char. Testa minutd, obliqua, triangulari, subcBquilaterali, IcBvi, crassd : postice 

 convexiore, marline crenulatd ; timbonibus prominentibiis. 



Shell minute, obliquely triangular, nearly equilateral, smooth, thick, and strong ; 

 posterior side the more convex : margin crenulate ; umbones prominent. 



Diameter, \\h of an inch. 



Localities. Cor. Crag, Sutton. 



Red Crag, Walton-on-the-Naze. 

 Clyde Beds. 



Recent, Mediterranean ? Britain, Hebrides. 



This species is very abundant in that rich Depot of small shells, in the Coralline 

 Crag at Sutton, and from its denticulated margin and prominent hinge teeth, the 

 valves are often found united. There are two different forms or varieties of it, one of 

 which is larger and rnore compressed, with the posterior side less rounded, correspond- 

 ing precisely with the recent shell, this is comparatively rare, while the other, (which, 

 from its greater difference in being smaller, longer, that is with a greater comparative 

 diameter from the anterior to the posterior margin, more tumid, and more rounded on 

 the posterior side, was thought to be a different species, and named subtrigona in my 

 Catalogue,) may be procured in large numbers. Until lately this shell was regarded as 

 a very rare species in the recent state, and is said now to be more plentiful in the seas 

 of North Britain than in the South, and the difficulty of obtaining a good series for 

 comparison in 1840, induced the belief of its distinction for the Crag shell. The variety 

 subtrigona, the common Crag shell, is roundedly triangular, having a diameter quite 

 as large, occasionally even more so when measured from one side to the other, than 

 from the umbo to the ventral margin, but in the other variety it is quite the reverse ; 

 the exterior is quite smooth, and the margin of the adult shell is crenulated on the 

 inside, with about two dozen obtuse denticulations. In all cases the young specimens 

 have the margins smooth, but there are others which have attained their full 

 magnitude, with the edge quite free from crenulations ; most of my specimens are 

 colourless, but some few have the reddish brown, with which the recent shell is 

 tinged only partially removed, leaving, as it were concentric bands of this colouring 

 matter. The ligament of this species is placed upon a somewhat prominent fulcrum, 

 readily distinguished externally when the valves are united, and not even partially 

 covered by an extension of the dorsal edge. 



