20 MOLLUSCA FROM THE CRAG. 



considered as belonging to any other ; there is, however, even in that state an irregu- 

 larity in the arrangement of the rays, which are at rather unequal distances, and have 

 indications of the squamse that so prominently ornament the adult shell. 



When full grown its true characters are displayed, and no mistake can, I think, 

 then arise respecting its generic distinction from that of Pecten. Our shell is nearly 

 of an oval form, having its height or diameter from the umbo to the ventral margin, 

 one sixth more than in a contrary direction. The lower or adherent valve is rather 

 the deeper of the two, while the upper or left valve is nearly flat ; the latter is orna- 

 mented with numerous undulating rays at somewhat irregular distances, amounting 

 to as many as forty in one specimen, and these are elevated and arched at different 

 periods of growth, giving a roughened file-like appearance to the exterior ; on the other 

 valve the rays are fewer, less regular, and the imbricated squamose appendages larger 

 and more prominent, resembling in that character the common Oyster. The hinge-line 

 in the adult shell is nearly straight, having an elongated and deep sulcus for the 

 ligament, enlarging downwards, and slightly visible externally at the umbo ; this is 

 placed a little on the posterior side of the centre, giving a slight inequality to the 

 auricles, the anterior one being of course the larger ; the muscle mark is of an enor- 

 mous size, occupying nearly half the length of the shell, while the diameter of it in the 

 other direction is rather less. 



In assigning the Crag shell to H. Dubuissoni, Mr. J. Sowerby says he was guided 

 to that determination by the description only, and having seen but one valve, thought 

 the comparison suited better with the shell from the older Tertiaries than with the 

 other species described by M. De France. I regret exceedingly not having been able to 

 obtain, for comparison, a specimen of either of the two species described by M. De France, 

 and the dependence here is also based upon an insecure foundation. I feel, however, 

 more disposed to refer our shell to //. Cortesgi, as well from what appears a greater 

 correspondence in their external characters having both valves for our guidance, as 

 also from the age of the formations in which they were obtained ; and as no new name 

 is imposed, the simple alteration is a matter of no great importance should it hereafter 

 be found to be erroneous. 



Pecten.* Pliny, Jldrov, &c. 



Pecten. Chem. 1784. Bolten, 1798. 



Pandora. Meyerle, 1811. 



Janira. Schum. 1817. 



Neithea. Drouet. 1824. 



Janera. G. B. Sow. Jr. 1842. — P. maximus. 



Pecten. Ritmphius, 1705. Browne, 1756. Chem. 1784. 



Ostrea (sp.). Linn. 



Argus (sp.). Poli. 1795. 



Chlamys (sp.). Bolten, 1798. 



* Etym. :— Pecten, a comb. 



