BIVALVIA. 255 



Fragments of this species are by no means rare, and pervade the whole of the Red 

 Crag Deposit. The specimen figured is one of a pair of valves found in situ at Walton 

 Naze, but in a very fragile condition, and much reduced in substance. The fragments 

 from Suffolk display, generally, a greater solidity in the anterior portion, which is 

 that most commonly obtained. 



Much importance has been placed upon differences in proportional dimensions. 

 In this, the length is about five times that of its height, while in S. siliqua some speci- 

 mens are as one to eight. The greatest difference appears to be in the impression of 

 the margins of the mantle ; in this it is more inward, or further from the anterior edge, 

 and, on the contrary, the more linear shell (8. siliqua} has the impression nearer the 

 extremity, with a slight difference also in the form of the anterior adductor. The 

 truncation of this extremity generally forms an angle of about 95, but this is not 

 constant ; and I am inclined to believe (although they are here separated in deference 

 to the recent conchologists, who have better materials to work upon) that the two 

 forms are merely varieties of one and the same species, the differences of locality and 

 other conditions producing all the variations shown by the two shells.* 



2. SOLEN SILIQUA, Linncem. Tab. XXV, fig. 7, a e. 



SOLEN SILIQUA. Linn. Syst. Nat., ed. 12, 1113, No. 34, 1767. 

 Poll. Test. Sicil., vol. i, pi. 10, figs. 711. 

 Turt. Brit. Biv., p. 80, pi. 6, fig. 5, 1822. 

 Phil. En. Moll. Sic., vol. i, p. 4 ; vol. ii, p. 5. 

 Forb. and Hani. Hist. Brit. Moll., vol. i, p. 246, pi. 14, fig. 3. 



NOVACULA. Mont. Test. Brit., p. 47, 1803. 



LIGULA. Turt. Brit. Biv., p. 82, pi. 6, fig. 6. 



MAJOR. List. Hist. Conch., lib. iii, fig. 255. 



Spec. Char. Testa lineari, recta, Icevigatd ; extremitate subtruncatd non marginatd ; 

 in valvd sinistrd unidentato, in alterd bidentato ; dentibus lateralibus elongatis. 



Shell linear or cylindrical, straight and smooth; extremity truncated, not mar- 

 ginated ; one cardinal tooth in the left valve, and two in the other ; lateral teeth 

 elongate. 



Length, 5 inches. Height, f inch. 



* It is possible that a portion of the Red Crag of Suffolk may have been derived from the destruction 

 of the Older or Coralline Crag Formation, intermixed with the exuviae of animals belonging to the seas of the 

 former period, as well as with other extraneous fossils. The cliff at Walton Naze, however, affords strong 

 presumptive evidence that the whole of the Red Crag is not derivative, and that the animals whose remains 

 are there deposited, lived and died in the spot where they are now found. Bivalves are frequently obtained 

 in this locality with the two portions united, and it seems scarcely possible that such a specimen as the 

 above could have been removed out of one Formation to have been deposited, with its two fragile valves 

 in their natural position, in the mud or sand of a succeeding period. 



