CLASS PELECYPODA 
79 
Genus CRENELLA Brown, 1827. 
Shell oblong-oval, equilateral, ventricose; beaks obtuse, slightly turned 
to one side; hinge destitute of teeth, but with a flattened, horizontal, 
slightly crenated plate on one side of the hinge in each valve; right valve 
with a triangular, horizontal, projecting, reflexed plate, and the left one 
with an oblique plait, both of which are a little crenated externally. 
(Brown.) 
Type. Mytilus decussata Montagu. 
Distribution. Low-water mark to 150 fathoms; Norway, Iceland, 
Greenland, New England, Britain, France, Pacific Coast. 
Range in time. In the Eocene and Pleistocene. 
Crenella decussata Montagu, 1808. 
Test. Brit. Suppl., p. 69, 1808; Forbes and Hanley, Brit. Moll. 2; pi. 45, fig. 2. 
Shell longitudinally ovate, with the umbo at the smaller end; sides 
equal. It is very thin, pellucid, of a pearly white when divested of the 
epidermis, which is of a pale olive brown; and is finely striated longitudi¬ 
nally, crossed by more minute striae in a transverse direction, that give 
it a decussated appearance when examined under a microscope. The 
inside is smooth with a nacred gloss; at the hinge is a slight indenture, 
the margin contiguous and slightly denticulated; near the front margin 
is a singular, reflected transverse ridge, but whether this is a constant 
character, or accidental, cannot be determined; indeed only one valve of 
this curious little shell has come under examination, and that through the 
favor of Mr. Laskey, who found it in sand on the Scottish coast. Length, 
about y 8 of an inch; and not quite so broad. (Montagu.) 
This pretty little species is very ventricose at the umbos, whence its 
convexity regularly and gradually diminishes in all directions. Its valves 
are thin and fragile, yet not particularly so for their minute size, and are 
covered with a dull ashy olivaceous or pale olive-coloured epidermis, 
beneath which the surface appears white. The very many raised and 
somewhat granulated striae, which radiatingly adorn the entire exterior, 
are all nearly equally strong, more or less divergent and very closely dis¬ 
posed, since the interstices, as they widen, become filled up with inter¬ 
mediate costellar striae; the granules are much crowded, and very minute. 
(Brit. Moll.) 
Type in British Museum? Type locality, Scottish Coast. 
Range. Bering Sea to Puget Sound and San Pedro, California. Also 
in the Atlantic. 
