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 characters, and it is there- 
fore thought necessary to unite the two. 
The typical form may be considered as fig. 5a, 4; 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 appear more distinct. The smaller variety, or pis?formis, 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, J. Sowerby. Tab. XVI, fig. 4a—d. 
ASTARTE OBLIQUATA. J. Sow. Min. Conch., t. 179, fig. 3, 1817. 
— — S. Wood. Catalogue, 1840. 
Spec. Char. Testd ovatd, transversa, subequilaterali, compressa, striatd, striis obliquis; 
umbonibus acutis prominulis ; lunuld lanceolata; margine crenulato. 
Shell ovate, transverse, slightly mequilateral, compressed and striated, striz 
oblique; beaks rather sharp, and somewhat prominent; an elongated lunule and 
margins crenulated in the adult state. 
Diameter, 1} 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 very 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 4. Burtinii, the species so common 
and so variable in the Coralline Crag. 
