Pvn H,i I'UitlS

122 MYTILIDiE.
and those on each side of it in the same family are
never capable of being precisely defined, it is obviously
undesirable to encumber our system of classification
with more divisions and names than are necessary to
separate and recognize the species
group.
contained in such
1. Modiolaria marmora'ta*, Forbes.
Mytilus marmoratus, Forb. Mai. Mon. p. 44. Crenella mctrmorata, F. & H.
ii. p. 198, pi.
xlv. f. 4.
Body thick, pale-yellow: incurrent tube large and bagshaped,
formed of two pendulous puckered flaps of the mantle,
mottled with purplish- or reddish-brown and white flakes ;
margin plain : ecccurrent (or anal) tube small and conical, of
the same colour as the larger tube, farrowed at the base, and
having the mouth or opening fringed
witb four or five minute
dark cirri : foot white, very long and almost cylindrical, with
a deep byssal groove :
byssus semitransparent but strong.
Shell oval, very gibbous and obliquely angular, rather
thin, glossy, and somewhat iridescent :
sculpture, 15-18 longitudinal
ribs on the anterior side and 20-25 on the other
side ; these ribs or striae occupy the two sides only ; the intermediate
space is smooth, with the exception of very fine and
crowded transverse lines, which traverse the whole surface
and give the ribs an appearance of being minutely punctured :
colour yellowish, irregularly mottled with purplish- or reddish-brown
spots or blotches, sometimes forming zigzag
streaks, occasionally of a uniform orange hue : epidermis light
green : margins rounded and obliquely truncate at the anterior
end, nearly straight and slightly gaping on the ventral
side, wedge-shaped or bluntly pointed at the posterior end,
whence they slope backwards with a gentle curve to the dorsal
angle or hinge-line : byssal sinus long and narrow : beaks
small, swollen, inflected (as in the genus Verticorclia), and
divergent, placed near the anterior margin: ligament thick
and strong, yellowish-brown, nearly concealed in the embrace
of the hinge-plate : hinge-line slightly curved, occupying the
whole of the dorsal margin : hinge-plate strengthened by an
internal rib and receiving the ligament in a shallow groove ;
* Marbled.
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