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 squamee 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 
hgament, enlarging downwards, and slightly visible externally at the umbo; this is 
placed a little on the posterior side of the centre, giving a slight imequality 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 ZZ. Cortesyi, 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. 
Pectren.* Pliny, Aldrov, &c. 
Pucotmn. Chem. 1784. Bolten, 1798. 
Panpora. Megerle, 1811. 
Jantra. Schum. 1817. 
NertHea. Drouet. 1824. 
Janpra. G. B. Sow. Jr. 1842.—P. maximus. 
Prcren. Rumphius, 1705. Browne, 1756. Chem. 1784. 
Osrrna (sp.). Linn. 
ArGus (sp.). Poli. 1795. 
Cutamys (sp.). Bolten, 1798. 
* Etym. :—Pecten, a comb. 
