GASTEROPODA. 183 



and air are speedily expelled, and the shell then becomes so firmly fixed by atmospheric 

 pressure, as to be removed with great difficulty. Shells of this form have early made 

 their appearance, and several have been figured from the Secondary formations. 



1. PATELLA VULGATA. Linn. Tab. XX, fig. 8, a b. 

 PATELLA VULGATA. Linn. Syst. Nat. p. 1258, 1758. 



VULGAEIS. Da Costa. Hist. Nat. Test. Brit. p. 3, t. 1, fig. 1-2, 1/78. 



VULGATA. Mont. Test. Brit. p. 475, 1803. 



Brown. Illust. Brit. Conch, pi. 37, fig. 15, 1827. 



Flem. Brit. An. p. 216, 1828. 



Lam. Hist, des An. s. Vert. 2d edit. torn. 7, p. 535. 



S. Wood. Catalogue 1842. 



Loven. Ind. Moll. Scand. p. 26, 1846. 



P. Testa ovato-comcd, depressd ; vertice eccentrico ; costis tenuibus, subangulatis, 

 imbricatis ; margine subdentato. 



Shell shield-formed, ovate, and depressedly conical, with an eccentric vertex, 

 covered externally with low subangulated and subimbricated rays ; margin slightly 

 crenulated. 



Longest diameter ', of an inch. 



Locality. Red Crag, Sutton. Recent, Britain. 



This species, so exceedingly abundant at the present day upon our own coasts, 

 appears to have been very scarce during the Red Crag period, as only two specimens 

 have as yet come into my possession ; these are small, and differ slightly from the 

 general character of the recent shell, in having the rays exceedingly rough or slightly 

 imbricated ; there is, however, I think, no doubt of its identity. 



CHITON. Linn. 1758. 



Gen. Char. Animal in a recent state covered with eight testaceous valves, generally 

 broader than long, more or less arched or angulated. These valves are fixed at their 

 lateral edges, in the coriaceous skin which forms a rim around them ; collectively, they 

 are of an ovate, or oblongo-ovate form, rounded at the extremities, one valve slightly 

 or partly overlapping the succeeding one, enabling the animal to roll itself up into a 

 ball ; each valve furnished with two projecting processes. 



As the eight valves (called by M. de Koninck cerami) of each individual in 

 this genus are held in position by the connecting integuments, they are in a fossil 

 state, as might be supposed, almost always detached and separated, at least in such 

 sandy formations as the Crag. The band which connects and surrounds these valves, 

 in the living state, is of a coriaceous nature, sometimes beset with prominent spines, 

 sometimes rugose like shagreen, and often nearly smooth. This is a good character for 

 specific distinction, but an auxiliary not to be obtained in Crag specimens ; the valves, 

 however, when well preserved, may often be distinguished by their sculpture, as well 



