BIVALVIA. 157 
crenulations. In the older shells the umbones are much eroded, and only slightly so in 
the younger specimens. ‘This probably arises from the one possessing more animal 
matter than the other. Old shells are more easily decomposed. 
Specimens from different localities present considerable differences from each other : 
those from Highgate are nearly smooth, except at the umbones. One specimen from 
Potter's Bar (fig. 13, d) is perfect at the umbo, which is much elevated, giving it a 
different outline ; but I think it is merely a variety (wmbonata). Figs. 13 a and / are 
from Sheppey ; they are irregularly ridged all over, and may perhaps prove to be distinct ; 
but the specimens I have seen are scarcely perfect enough for fair determination. Fig. 9, 
Tab. XXIV, is from Hampstead, and is considered by Mr. Edwards as a distinct species 
—called by him A. filigera; but the specimen is not in good condition, and I have 
placed it here as a variety for the present, though I suspect when better specimens are 
found they will deserve to be separated, as the ridges are very distinct and prominent. 
Figs. 11 a, 6 represent two specimens from Alum Bay, out of the bed which is there 
considered to be the attenuation of the London Clay. These specimens are fixed upon 
the matrix, and I am unable to see the interiors. In this genus, in which specimens 
are so very variable, even when perfect, I am unwilling to do more than consider them as 
varieties, and this latter I will call 4. rugata, var. subrugata. 
A, AstarTE TENERA, J. Sowerby’s MS. Tab. XXIV, fig. 14 a—c. 
ASTARTE TENERA, J. Sow. MS., Morris. Catal. Brit. Foss., p. 80, 1843. 
— — Morris. Journ. Geol. Soc., vol. viii, p. 265, pl. xvi, fig. 6, 1852. 
—_ — Td, Catal]. Brit. Foss., 2nd edit., p. 188, 1854. 
Spee. Char. “A. Testa subtrigond, depressd, tenerd, inequilaterali, concentrice, 
erregulariter rugosa ; latere postico compressiusculo ; umbonibus submedianis ; lunuld ovata, 
profundd, levigatd ; marginibus crenulatis.’—Morris. 
Shell subtrigonal, depressed, thin, inequilateral ; concentrically but irregularly striated ; 
siphonal region slightly compressed, and angulated ; beaks subcentral ; Iunule ovate, deep, 
smooth ; margins crenulated. 
Length, 1} in.; height, 1} in. 
Locality. Herne Bay. 
The exterior of this shell m the young state is regularly ribbed or sulcated in the 
directions of the lines of growth, becoming smooth, or irregularly so, on the older half, 
giving the surface in some specimens a ridgy appearance. ‘The shell figured by 
Mr. Morris is an adult individual—presumably so by its having a crenulated margin. 
I have not been able to see the very young condition of this species, but some specimens 
have the margins smooth, from which I imagine they are immature. The specific name 
