BIVALVIA. 295 



employed ; but whether the shell is the instrument used for such a purpose is not so 

 satisfactorily determined. We sometimes find specimens with the asperities of the 

 shell worn down, where the animal has located itself in mud ; and at other times 

 specimens have never had their roughened surfaces at all abraded. We know also 

 that other animals than those belonging to the Mollusca, such as have no rough or 

 hardened exterior, are able to penetrate deeply into stone as well as into wood. 



I. Pholas cylindrica, J. Sowerby. Tab. XXX, fig. 8, a — d. 



Pholas cylindrica. J. Sow. Min. Conch., t. 198. 



_ _ Nyst. Coq. Foss. de Belg., p. 41, 1844. 



Dale. Hist, and Antiq. of Harwich, p. 295, t. 13, fig. 6, 1/30. 



Spec. Char. Testa elongate!, subcylindricd, temii, fragili, valde inaquilaterali ; latere 

 antico abbreviato, postico porrecto, accuminato ; costatd, costis anticis dentato-mtiricatis> 

 costis posticis ad extremitatis evanescentibus. 



Shell elongate, subcylindrical, thin, and fragile, very inequilateral ; anterior side 

 short, posterior prolonged, and obtusely pointed ; costated ; ribs on the anterior side 

 toothed and rough, and on the posterior obsolete towards the extremity. 



Length, 3 inches. Height, I inch, nearly. 



Locality. Cor. Crag, Sutton. 



Red Crag, Sutton and Walton Naze. 



This species is very abundant at Walton-on-the-Naze, but, from its fragility, speci- 

 mens are difficult to obtain in perfect condition. In the Coralline Crag I have met 

 with only a few fragments. 



It most nearly resembles the British species, P. parva, from which it may, however, 

 be distinguished by its being more inequilateral, the siphonal side occupying at least 

 two thirds of the entire shell, and the opening for the foot on the ventral portion of 

 the anterior side is larger and deeper. The shell is reflected over the back, covering 

 the umbones, but the reflected portion is not partitioned like that of Ph. dactylus, and 

 there is a tubercle at the middle, flattened by the pressure of the valves ; the large 

 and subovate impression of the adductor muscle is placed about midway between the 

 umbo and the posterior extremity, and the line of the mantle-mark extends inwards 

 a little beyond it; the plate or tooth curves from immediately beneath the beak, and is 

 slightly spatulate in form. There is a small opening in the dorsal portion of the 

 anterior side, which was no doubt covered by an accessory piece. Fragments of such 

 a valve have been found by myself at Walton, most probably belonging to this species, 

 (fig. 8, d.) 



The figure given by Dale, above referred to, is no doubt our shell, as he says Dr. 

 Woodward found it in Harwich Cliff; and as it is abundant at Walton Naze, its 



