BIVALVIA. 179 



6. AsTARTE MUTABiLis, S. JFood. Tab. XVI, fig. 1 a — /i. 



AsTAKTE MUTABILIS. S. TFood. Catalogue, 1840. 



— PLANATA. Ni/st, (not Sowerbjr.) Rech. Coq. Foss. d'Anv., p. 7, No. 2,3, 1835. 



— — LI. Coq. Foss. de Belg., p. 151, pi. vii, fig. a— 6, 1844. 



Spec. Char. Testa transversa, ohlongd, subcordatd, crassd, ineeqtiilaterali, postice 

 longiore, truncatd ; natibus late sulcatis ; Immld profunde excavatd ; lavigatd ; marginibiis 

 incrassatis, cremdatis. 



Shell transversely oblong, inequilateral, slightly heartshaped, thick and strong; 

 posterior side the longer, and truncated ; umbones broadly sulcated ; lunule smooth, 

 and deeply excavated ; margins thickened and crenulated. 



Length, 2 inches ; height, l|-ths ditto. 



Localities. Cor. Crag, Ramsholt, Sudbourn, Gedgrave, Sutton. 

 Red Crag, Sutton, Alderton, Baldersey. 

 Mam. Crag, Bridlington, {Leckenbg.) 



This is one of our most changeable species in this variable genus, and the two 

 forms that have been figured are the extremes of variation, fig. «, b, being the most 

 constant, may be described as the type of the species. The adult shell is particularly 

 thick and strong, with the muscle marks deeply indented ; the posterior one is large 

 and nearly oval, the anterior one is rather less, and more straightened on the inner 

 side, and immediately beneath the anterior lateral tooth is the third small muscular 

 impression ; the line of the mantle indicates posteriorly the short siphons of the living 

 animal ; externally the shell is nearly smooth, except at the umbones, wliich are nearly 

 flat in the adult shell, with about a dozen broad and deep furrows ; the umbones are 

 generally sharp, curving over a broad and deeply-excavated lunule, with a large and 

 elongated corselet, both of which are quite naked or smooth ; in the right valve the 

 cardinal tooth is large and projecting, somewhat rugged at the sides, with the like 

 striations upon the inner surfaces of the two teeth in the left valve ; lateral teeth 

 obsolete ; the ventral margin is nearly straight, and the posterior side somewhat 

 quadrate. The margin of this species is quite smooth and sharp until it has attained 

 its full size, when it becomes much thickened and deeply crenulated. Numerous 

 specimens of the young shell may be obtained at Sutton, and I have traced them 

 down to less than a line in diameter. 



Fig. 1 e,/is, I presume, only an aberrant form of this species; it was obtained at 

 Bridlington by Mr. Leckenby, to whom I am indebted for permission to have it 

 figured. 



