110 MOLLUSCA FROM THE CRAG. 



This is a most abundant, shell at Clacton and Stutton, and is subject to a 

 good deal of variation, both in the outline and in its exterior ornament ; in all varieties 

 the young shell is generally flatter or more compressed than when full grown ; there 

 is also a difference in the substance of the shell, some specimens are thin and tender, 

 while others are quite thick and strong. Those which are most flat are also in 

 general thinner, and have a greater length from the anterior to the posterior, and are 

 longer also on the hinder side. The specimens from Grays are mostly the thicker 

 variety, in which the posterior side is remarkably short and truncate, and the striae on 

 the outside are finer and more numerous : this has been called P. sulcatum (fig. 1, b), 

 l)ut it is, I believe, no more than a variety, as a recent acquisition of numerous 

 specimens show every intermediate alteration to those which are much less inequi- 

 lateral, and have more distant ridges upon the exterior. These fossils seem to present 

 rather more difi"erences than any specimens that I have seen of the recent shell, and it 

 is, therefore, thought desirable to have the two extreme forms represented, in order 

 more efi"ectually to display these variations. The hinge is furnished with two cardinal 

 teeth in each valve, one small and simple, the other large and bifid, the posterior one 

 in the right is bifid, while in the left valve it is the anterior ; there is a large promi- 

 nent lateral tooth before and behind the umbo at nearly equal distances in the left 

 valve, and two on each side in the right : this hinge line is broad with teeth of cor- 

 responding magnitude in the thick variety, and in some specimens these form with the 

 umbo an angle of little more than 90°, whereas in others of the elongated variety that 

 angular line will be as large as 130". In the thick variety, the posterior side projects 

 but veiy little behind the umbo, nearly the whole of the animal being on the anterior 

 side of the shell ; in other specimens, this side is two fifths as large as the other, and 

 the shell much less inequilateral. Similar differences may be also obser\'ed in tlie 

 sculpture of the exterior, they are always ornamented more or less with concentric 

 or elevated lines of growth, but in some, these markings are numerous, rounded, and 

 placed close together, while in others they are sharp and narrow with a considerable 

 plain concave space between them. 



It is a common shell in England, in the living state, and has a wide Geographical 

 distribution in Europe, extending from Sicily to Sweden. 



A few specimens also of this species were obtained by Capt. Alexander from the 

 Mam. Crag. 



2. PtsiDiUM Henslowianum, S/ieppnrd. 



Tellina Hexslowiana. Shep. Trans. Linn. Soc, vol. 1.1, p. l.")0, 182."). 



Cyclas appendiculata. Tiirt. Man. Land and F.-\V. Shells, pi. 1, %. 6. 



Peba appesdiculata. Leach, I\1SS., fide Jemjns. 



PisiDiUM He.nslowianum. Jenyns. Trans. Cam. Phil. Soc, vol. iv, p. 308, t. 21, figs. C — 9, 1831. 



— — Graij. Man. Land and F.-W. Shells, p. 28.i, pi. 1, fig. (>. 



— — Forb. and Haul. Hist. Brit. Moll., vol. ii, p. 131, pi. 37, fig. 1 1, 1849. 



