BIVALVIA. 189 



This shell is so excessively variable, and presents such extraordinary differences in 

 character, that when my Catalogue was compiled, the smaller variety was considered 

 as a distinct species, and passed under the MS. name of pisiformis, given to it by my 

 friend the late Rev. G. R. Leathes, in consequence of its pea-like form, which this 

 small variety assumes when the two valves are united ; a large additional number of 

 specimens since obtained are found to possess intermediate chai'acters, and it is there- 

 fore thought necessary to unite the two. 



The typical form may be considered as fig. 5«, b ; this is a large shell, inequilateral 

 and somewhat compressed, and slightly angular on the posterior side, and if found 

 alone, would most deservedly be separated from the smaller and more globose variety, 

 as two shells cannot well cqypear more distinct. The smaller variety, or jnsifoniiis, has 

 a height greater than its length, and the posterior side is nearly convex ; the lunule is 

 large and smooth, and the shape of it is modified by the form of the shell. The ex- 

 terior is covered with ridges, broad and smooth, having only a narrow line between 

 them ; the ridges pass over the shell obliquely, from the edge of the corselet round to the 

 edge of the lunule, and they take their rise on the posterior side, where they are closer 

 and more numerous, running out or disappearing at the margin of the shell, the lines 

 of growth, of course, crossing them at a variable angle ; the number of these ridges 

 vary according to the age or size of the specimen, the very minute not possessing 

 more than half a dozen, while seventy may be counted in the larger and full grown 

 individuals. 



A specimen from Thorigny, given to me by Sir Charles Lyell, very much resembles 

 our shell, and is probably only a variety. 



17. AsTARTE OBLiQUATA, /. Soioevby. Tab. XVI, fig. A a — d. 



AsTAETE OBLiauATA. /. Sow. Miu. Conch., t. 179, fig. 3, 1817. 

 — — S. Wood. Catalogue, 1840. 



Spec. Char. Testa ovatd, transversa, sub(squilaterali, compressd, striata, striis obliquis; 

 umbonibus aciitis prominulls ; lunula lanceolatd; margine crenulato. 



Shell ovate, transverse, slightly inequilateral, compressed and striated, striae 

 oblique ; beaks rather sharp, and somewhat prominent ; an elongated lunule and 

 margins crenulated in the adult state. 



Diameter, \\ inch. 



Localities. Red Crag, Walton-on-the-Naze, Sutton. 



This shell is very abundant at the former locality ; I have very rarely met with it 

 at any other. 



As far as can be determined by the figures and descriptions of the Belgian authors, 

 it is veiy doubtful if this species has been found in that part of the world ; the shell 

 so called by M. Nyst appears to be only a variety of A. Burtinii, the species so common 

 and so variable in the Coralline Crag. 



