BIVALVIA. 155 
lated edge of the shell as probably analogous to the outer lip of some univalves, where 
the margin of the aperture is denticulated only in the adult state. 
This genus does not appear to be present in the Hocene Formations of France; in 
our own country, I know it only from that portion of the Lower Tertiaries which is 
inferior to the Bagshot Sands, with the somewhat doubtful exception of the shell figured 
as Astarte modicella. 
The Crag shells of this genus, though of thick and solid texture, are frequently 
perforated by some of the zoophagous feeders, but they seldom show any erosion at the 
umbones. Both these actions appear peculiar to some deposits only. Some of the older 
Tertiary specimens are much eroded at the umbones, while others of the same species 
from another locality are not at all so. 
1. Asrarte Cuarenponensis, Hdwards’ MS. Tab. XXIV, fig. 17 a, 4. 
Spec. Char.  Testé magna, crassissimd, irregulariter quadrangulata, tumidiusculd, 
sublevigatd, valde inequilaterali ; pedi-regione brevissind, subtruncatd, alterd longiore, 
subquadratd, compressiusculd ; impressionibus muscularibus profundis ; marginibus crenu- 
latis. 
Shell large, very thick and strong, irregularly quadrangular, slightly tumid, and 
nearly smooth; very imequilateral; umbones depressed, very excentric; pedal region 
short, anal side truncated, and somewhat compressed ; muscular impressions deep ; 
margins crenulated. 
Length, \3 ich; height, 14 inch. 
Locality. London Clay: Clarendon. 
This species has yielded to Mr. Edwards a great number of specimens, but I do not 
see among them a very young individual, although one has its margin smooth, and as 
such I presume it is not full grown. he surface where perfect appears to have been 
nearly smooth, showing only very fine lines of growth. The umbones of all the speci- 
mens have been eroded, and I am unable to see if the very young state of the shell was 
covered with ridges. The shell has a tolerably large lunule not very strongly defined, 
but this varies according to the tumidity of the individual specimen. The proportional 
dimensions show considerable variation. 
