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 [uinhonatd). Mgs. 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. filicjera ; 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. 

 Pigs. W a,h 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 A. rugata, var. suhrugata. 



4. AsTARTE TENERA, /. Soioerbg'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, pi. xvi, fig. 6, 1852. 



— — Id. Catal. Brit. Foss., 2nd edit., p. 188, 1854. 



Spec. Char. " A. Testa mhtrigond, depressd, tenerd, inaquilaterali, concentrice, 

 irregulariter rugosa ; latere postico compressiusculo ; umbonibus submedianis ; lunula ovatd, 

 profunda, lavigatd ; marginihus crenulatis." — Morris. 



Shell subtrigonal, depressed, thin, inequilateral ; concentrically but irregularly striated ; 

 siphonal region shghtly compressed, and angulated ; beaks subcentral ; lunule ovate, deep, 

 smooth ; margins crenulated. 



Length, \\ in. ; height, \\ in. 



Locality. Heme Bay. 



The exterior of this shell in 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 



