Pvn H,i I'UitlS

MODIOLARIA. 125
2. M. costula'ta *, Risso.
Modiolus costidatus, Risso, Hist. nat. l'Eur. rnerid. iv.
p. 324,
Crenella costidata, F. & H. ii. p. 205, pi.
xlv. f. 1.
f. 165.
Shell oblong, narrower at the anterior than at the opposite
about 10 ribs
end, convex, rather thin, and : glossy sculpture,
on the anterior side and twice as many on the other side ;
middle area somewhat depressed, destitute of ribs, but the
whole surface of the shell is marked by very numerous and
wavy transverse striae, which gives it a silky appearance :
colour cream-white, beautifully mottled with rich purplishbrown
streaks often arranged in a zigzag pattern : epidermis
thin, light green : margins rounded at the anterior end, nearly
straight or slightly incurved on the ventral side, broad and
rounded at the posterior end, and forming a gentle curve on
the dorsal side : beaks small, inflected and divergent, not so
much swollen as in M. marmorata, nor placed so near the
anterior : margin byssal sinus, ligament, and hinge-line
that : species hinge-plate strengthened by
as in
an internal rib or
ledge and strongly toothed ; ligamental groove narrow : hinge
reflected and slightly indented or notched : inside highly na-
creous, and of a deep purplish-brown except at the edge,
which is finely crenulated on the anterior and posterior sides :
scars indistinct. L. 0*4. B. 0-2.
Habitat : Under stones and among
small sea-weeds
in rock-pools at low spring-tides on the south coasts of
Devon and Cornwall ; Herm (Macculloch) ; Oxwich Bay,
near Swansea (J. G. J.) ; and Bundoran, co. Donegal
(Waller). It is rather local, but not uncommon. Searles
Wood has found it in the Red and Coralline Crag. It
appears to be a southern form, and to range from Brittany
(Mace) to Sicily (Philippi) and the Canaries (Webb and
Berthelot) .
This very pretty shell differs from M. marmorata in
being less gibbous or tumid, much narrower, and of a
brighter and more variegated colour; the posterior
margin is regularly curved instead of obliquely wedge-
* Slightly ribbed.