278 MOLLUSCA FROM THE CRAG. 



MYA TRUNCATA. Middendorff. Malacozool. Rossica, p. 585, t. 19, figs. 13 15, 1849. 



_ _ Lyell. Trans. Geol. Soc., vol. vi, 2d series, p. 137, pi. 17, figs. 5, 6, 1839. 



_ Forbes. Mem. Geol. Surv., vol. i, p. 408, 1846. 



Hancock. An. and Mag. Nat. Hist., vol. 18, p. 337, 1846. 



var. PELAGICA. King. An. and Mag. Nat. Hist., vol. 18, p. 242. 



OVALIS. Turt. Brit. Biv., p. 33, pi. 3, figs. 1, 2, 1822. 



PULLUS. J. Sow. Min. Conch., t. 531. 



SWAINSONI. Lovcn. Iiid. Moll. Scand., p. 49, 1846. 

 SPHENIA SWAINSONI. Turt. Brit. Biv., p. 37, pi. 19, fig. 2, 1822. 

 CHAMA TRUNCATA. Da Costa. Brit. Conch., p. 233, pi. 16, figs. 1,1. 



PHOLAS LATUS. List. Hist. Conch., pi. 482, fig. 269. 



Spec. Char. Testa transversd, subovatd, in&qnilaterali ; antice rotundatd, tumidd, 

 clausd; postice truncatd, hiante ; cardinis dente porrecto, rotundato. 



Shell transverse, subovate, inequilateral ; anterior side rounded, inflated, and closed ; 

 posterior truncated, and gaping; hinge with one large, rounded tooth. 



Length, 3^ inches. Height, 2^ inches. 



Locality. Cor. Crag, Ramsholt, and Gedgrave. 

 Red Crag, Button. 



Mam. Crag. Chillesford, Bridlington. 

 Clyde Beds, Bracklesham, (Dixon). 



Recent, British and Scandinavian Seas, Behring Straits. 



This shell does not appear to have been rare in the Coralline Crag, the oldest 

 Formation from which I have seen it ; nor is it scarce in the Red Crag ; and in the 

 more recent Deposit at Chillesford it may be abundantly obtained in great perfection 

 with the valves in their natural position. Mya pullus is the young of this species, and 

 not of arenaria, as supposed by the authors of the ' Hist, of Brit. Moll./ vol. i., p. 172. 

 In my ' Catalogue/ I had assigned this as an identity with M. ovalis, Turt., and having 

 found this shell in great profusion at Butley, not exceeding it in size, I considered it 

 then as distinct. 



This species is subject to considerable variation, more especially in regard to its 

 length : those I have met with from the Coralline Crag are all of the longer variety ; 

 in the Red Crag the long and short are both obtained, but I have seen none there so 

 short and obliquely truncated as the variety found fossil at Uddevalla ; and in the 

 Clyde Beds.* At Chillesford, all the specimens I have seen are of the longer variety ; 

 that is, with the siphonal side somewhat elongated, while, perhaps, it is somewhat 



* The difference in the form of the mantle-mark in this variety was considered by the late Mr. G. B. 

 Sowerby as a character of sufficient importance for a distinct position, and Mr. Smith, in consequence, 

 proposed for it the name of Uddevallensis ; but this mark is merely the result of the shortened side of 

 the shell. 



It is now well known that many of the species inhabiting the Arctic seas exhibit great variation, and 

 still more abnormal forms, than are presented by the varieties of this species, have been recently obtained 

 from that part of the world. 



