Pvn H,i I'UitlS

DIPLODOXTA. 253
think that the coating arises from a deposit of faecal
matter in still and deep water, and not from any peculiar
secretion of the animal, or from a mineral or che-
mical deposit.
It is the Cryptodon rotundatiim of Searles Wood's
'
Catalogue of the Crag Mollusca/ Judging from an
examination of a single valve so named by Forbes, his
Kellia abyssicola from the iEgean appears to be only
the decorticated state of the present species.
Genus IV. DIPLODON'TA *, Brown. PI. V. f. 7.
Body nearly globular, but : compressed mantle thick, having
a : plain margin tubes none : gills two on each side : labial
palps the same : foot lancet-shaped.
Shell squarish, convex, and smooth: beaks not much recurved:
h.mule inconspicuous : ligament quite external : teeth,
two cardinals in each valve, the anterior of which in the right
valve is simple, and the other bifid or cloven, the contrary being
the case in the left valve ;
laterals laminar and indistinct.
The peculiarity of this genus consists in there being
two separate gills and as many labial palps on each side,
instead of a single gill and palp as in all the other genera
of the same family, and also of the ligament being alto-
gether external, and the cardinal teeth more numerous
and symmetrical. In adult specimens the anterior
muscular impression is elongated, like that of Lucina.
The Diplodonta inhabit every part of the marine
world ; and many fossil species have been described.
Torell has lately dredged at Spitzbergen a new species,
which is nearly allied to the only one we have in the
British seas.
It is the genus Mysia of Leach, and (according to
AYoodward) Spharella
of Conrad.
* Double-tooth.