BIVALVIA. 17 



2. OsTREA PRINCEPS, S. Wood. Tab. I, fig. 1, a—d, and Tab. II, fig. 2, a—b. 



OsTREA UNDHLATA. Nyst. Coq. Foss. de Belg., p. 32-4, pi. 24, fig. 7 a, and pi. 26, fig. 7 b, 

 1844, (not J. Sowerby). 



Spec. Char. Tenia magna, crassd, rotundatd ; valvd sinistra convexd, cosfatd, cost is 

 numerosis elevatis, radiantibus, divaricatis, concentrice undato-pUcatis ; valvd superiori plana, 

 obsolete costatd. 



Shell large, thick, and rounded; left or lower valve convex, ornamented with 

 numerous, elevated, radiating and bifurcating costse, lines of growth or concentric 

 laminae of an undulate or waving form, slightly reflected over the ribs ; upper valve 

 flat, with faint depressed, or nearly obsolete rays. 



Diameter, five inches. 



Locality, Cor. Crag, Sutton, Ramsholt, and Sudbourn. 

 Red Crag, Sutton, Newbourn, and Bawdsey. 



This elegant shell was obtained by my friend, W. Whincopp, Esq., of Woodbridge, 

 who has kindly permitted me to have it figured ; a similar specimen, though not quite 

 m so great perfection, is in the handsome museum recently erected by the liberal 

 inhabitants of the town of Ipswich ; two or three more of the same dimensions are in 

 the possession of E. Acton, Esq., of Grundisburgh, and these with another in the 

 Cabinet of J. S. Bowerbank, Esq., constitute the whole that I have seen of that 

 magnitude. 



Such specimens appear to be very rare, as it was never my lot to find so large a 

 one during my researches in the Crag of Suff'olk, although the same shell, in its 

 younger state (fig. 2 t)), has been for many years in my cabinet, and from its presenting 

 characters in that condition, by no means distinct, it was considered only as a variety 

 of edttlis, but the deeply sculptured markings, so well displayed in the specimen 

 figured, seem to justify its being considered as a different species, although it 

 must be confessed, the gradations of alteration between the young of this, and some 

 of the varieties of the common edible species, are so trifling, that the line of 

 separation cannot satisfactorily be pointed out. There are, in this genus particularly, 

 perplexities in specific determination, so that no diagnosis can be given of any one 

 species that is not liable to serious deviation, and the present name is assigned 

 provisionally, from the apparently marked difi^erence in the adult state, and which, if 

 it be not specifically distinct, its elegance, a rather uncommon character in this genus, 

 will entitle it to be ranked as a marked and peculiar variation, worthy of a dis- 

 tinguishing appellation. 



A slightly sinuated form is visible on the posterior side, where the costse also 

 appear more particularly to have a divaricating character. Upon the young shell the 

 radiating ridges or costee are but few in number, and by no means prominent or 

 distinct, appearing then strongly to resemble the edidis ; as it enlarges, the difi'erences 

 become more visible, the rays being prominent and regular. The edge or inner margin 

 of the shell is deeply indented with a slightly reflected edge, thus producing the sub- 



3 



