THECIDEA. 13 



what is seen in C. Egnabergensis. I have obtained from the chalk at Gravesend 

 specimens of this species from the dimensions of a quarter of a line in diameter, to that of 

 three lines ; in these various specimens, the apex was likewise more or less sub-marginal, 

 in some almost central. 



C. Egnabergensis has been found in the upper and lower chalk of various localities, 

 such as at Northfleet, Kent, by Messrs. Bowerbank, Morris, Woodward, myself, and many 

 others; in that of Dover and Folkstone by Mr. Mackie, in the Sussex Chalk by 

 Messrs. Catt and Dixon, in that of Norwich by Messrs. Woodward, C. B. Rose, Fitch, &c. ; 

 on the continent this species is found at Meudon, Fecamps, Vendome (Loir et Cher), in 

 France, but never at Hampton Cliff in England, as stated by M. D'Orbigny, whose figures 

 of this species do not seem to have been drawn from very good specimens, the muscular 

 impressions not being well characterised. 



Plate I, figs. 8, 8". From the collection of Mr. Bowerbank. 

 fig. 8 e . Enlarged. 



„ fig. 8". Interior of attached valve, enlarged. 



„ fig. 8 6 . ,, upper valve ,, 



„ fig. 81 Enlarged figure, shewing both valves united and attached to a coral. 



,, fig. 9. From the collection of Mr. Mackie. 



„ fig. 9 a . Enlarged. 



„ figs. 10, 11. Two young specimens from Gravesend. 



„ fig. 12. Elongated malformation, C. oval-is, Woodward. 



„ fig. 13. A specimen attached by a great portion of its lower valve to a 



iSpatangus from Gravesend. 



„ fig. 13 a . Enlarged figure. 



„ fig. 14. Illustrating the profile of the same with both valves united, enlarged. 



Genus — Thecidea, Defrance. 1828. 



Animal with the mantle-lobes disunited, and adhering closely to the valves. Shell free, 

 or attached by the larger valve, oblong, or transversely oval, more or less irregular, 

 thickened, especially round the margin, structure perforated. Dental, or largest valve, 

 partially or entirely attached by its own substance; or when young, in some species, by a 

 pedicle issuing from the extremity of the beak; upper or unattached valve always less 

 convex than the dental one ; surface smooth, or otherwise ornamented ; hinge line more or 

 less straight, with two strong teeth in the attached valve, adapting themselves into cor- 

 responding sockets in the smaller valve ; beak more or less produced, with a well-defined 

 area and deltidium. Interior of valves variable : in larger valve a longitudinal, central, and 

 two lateral ridges are generally more or less visible, under which two deep muscular 

 impressions are seen : upper valve furrowed more or less deeply and regularly, to receive 



