BIVALVIA. 63 



TRIGONIA IMBRICATA, Sow. Tab. VI, fig. 8, 8a. 



TRIGONIA IMBBICATA, Sow. Min. Con., t. 507, f. 2, 3. 



Under this name, Mr. Sowerby has figured apparently an immature or young state 

 of a species of Trigonia from Ancliff, of which the adult specimens have scarcely been 

 recognised ; this small form is shewn in the figure Sa ; we believe, however, that the shell 

 represented by fig. 8 belongs to a more advanced stage of growth ; the peculiar imbrication 

 of this species noticed by Mr. Sowerby appears to arise from the erosion of the concentric 

 spinose tubercles which ornament the shell. 



The young stage of this shell is described in the 'Mineral Conchology,' as being 

 " Transversely oblong, depressed ; with five or six concentric, dentated, subimbricated 

 keels upon the rounded anterior side ; posterior side obliquely truncated, ribbed. The 

 carinee upon the surface of this little shell resemble terraces one above the other ; each is 

 divided into four or five angular lobes." 



Localities. Minchinhainpton, (fig. 8, in the British Museum Collection ;) Ancliff, 

 Wiltshire. 



CARDIUM, Linn. 1758. 



Gen. Char. Shell equivalve subcordiform, umbones prominent, contiguous. Hinge 

 with two cardinal and two lateral teeth in each valve ; the cardinal teeth are approximate, 

 oblique, crucially inserted, one with the other, lateral teeth remote. 



CARDIUM SEMICOSTATUM, Lycett. Tab VII, fig. 6, 6, 6. 



CAKDIUM SEMICOSTATUM, Lycett. Annals Nat. Hist., 1850. 



Testa parvd, ovato-orbiculari, convexd, umbonibus magnis, mediants, concentrice et tenuis- 

 sime striatis ; latere postico compresso, costulis crebris radiantibus decussatis. 



Shell small, ovately orbicular, convex, umbones large, mesial ; concentric striaj regular 

 and faintly impressed ; the posterior side is compressed, its concentric strise are crossed 

 by radiating closely arranged ribs. 



The figure of this little species is wide towards the ventral border and narrow towards 

 the umbones, the lunule is small or nearly obsolete ; the convexity of the valves is 

 moderate, and the flatness of the posterior side produces at its junction with the dorsal 

 surface a well-defined oblique angle. It would seem to be rare in the shelly beds of the 

 Great Oolite, but the Bradford clay of Wiltshire produces numerous casts of a Cardium, 

 which we believe to belong to this species, and which attained a much greater development 

 of growth ; casts of this shell are also abundant adjacent to the Tetbury Road Railway 

 station, a locality which is very prolific of the fossils of the Bradford clay. 



The height and lateral diameter are about equal, varying from 2i to 5 lines. 



Locality. Minchinhampton Common in the Great Oolite. It occurs likewise in the 

 middle division of the Inferior Oolite of the same district. 



