Pvn H,i I'UitlS

and conical ;
CRENELLA. 131
and the foot is a very extraordinary organ.
This is formed of two parts : one is the stalk, of a cylindrical
form \ and from the npper end of it, as if from a
sheath, issues a tongue-shaped
disk which serves for
crawling. The animal does not spin a thick byssus,
like Modiolaria, but secretes only a single slight thread
or filament as a point of attachment, and by means of
which it holds itself suspended in the water.
Herrmannsen supposed that the genus was synonymous
with Limopsis and belonged to the Arcidce. Mac-
gillivray placed it with Lima and Anomia in his family
Pectinina.
1. Crenella rhom'bea"*, Berkeley.
Modiola rhombea, Berk, in Zool. Journ. iii. p. 229. C. rhombea, F. & H.
ii.
p. 208, pi.
xlv. f. 3.
Shell obliquely rhomboidal, gibbous, rather solid, somewhat
glossy
and iridescent :
sculpture, 60-70 fine longitudinal ribs,
crossed by 12-15 transverse plates ; the former radiate from
the direction of the beaks, and occasionally bifurcate or
branch off towards the margin; the latter form imbricated
ridges, and are stronger on the posterior slope : colour pearlwhite
: epidermis extremely thin and easily rubbed off, pale
yellow : margins truncate or obtusely
rounded on the anterior
side, slightly incurved in front, produced into a semicircular
lobe on the posterior side, and forming a wing or arched crest
behind: byssal
sinus small: beaks globular and prominent,
placed close to the anterior margin, minutely striate in a
transverse direction, but not sculptured in any other way:
ligament narrow but thick, yellowish-brown, occupying about
two-thirds of the hinge-line : hinge-line straight :
hinge-plate
rather broad and strong, finely crenulated: hinge furnished with
a rather large wedge-shaped serrated tooth in each valve, one of
which interlocks within the other : inside nacreous and glossy,
inner margins
showing distinctly the impression of the ribs ;
notched all round : muscular scars very slight. L. 0*13. B. 0-2.
Habitat :
Rocks and gravelly bottoms, from low-
* Rhomboidal.