MYTILIDA. 265 
~ Shell oval or oblong. thin, moderately inflated, with terminal, or 
very nearly terminal, incurved beaks, surface finely or radiately 
striated, somewhat ‘str onger anteriorly, producing a distinct 
crenulation at the margin ; hinge edentulous, ligament thin, long, 
situated in a fine marginal furrow of the internal side ; muscular 
scars distinct, the anterior slightly smaller than the posterior, 
pallial impression simple. 
Conrad says that Deshayes’ Med. radiolata (Paris foss., 2d ed., 
vol. ii, p. 22) is congenerie with the above species. Its relation 
to the short, oval species of Modiola is very great, but the well- 
marked and comparatively large anterior muscular sear, internal 
ligament, and fine radiating striation may serve as distinction. 
In general character Arcoperna strongly recalls the type of 
Phaseolicama. It is not improbable that the recent Lith. cinni- 
mominus, Chemn., is a recent representant of Arcoperna, while, 
on the other hand, cretaceous species, like Mytilus pileopsis, 
d’Orb., and a few others, are equally correctly referable to it, as 
to Crenella, or to any of the allied genera. 
Myrina, H. and A. Adams, 1857. 
Distr.—M. pelagica, Forbes (exxix, 21). On floating blubber, 
off the Cape of Good Hope. 
Shell transverse, oblong, subequilateral, close, smooth, covered 
by a corneous epidermis; nacreous within; beaks submedian ; 
hinge edentulous; ligament internal, linear; anterior muscular 
impression large. 
Animal byssiferous, mantle open. 
SuspFaAMILy DREISSH NSIN. 
Anterior muscular scar resting on a thickened plate near the 
apex of the shell, hinge sometimes with an obsolete, long tooth, 
fulcra of ligament strong ; pullial line entire, rarely sinuated, 
The animal has closed mantle and short siphons, and would 
therefore go into the order Siphonida, suborder Integripalliata ; 
but the mollusk and its shell are otherwise too closely related to 
Mytilus to admit of such a separation. 
DreissensiA, Van Beneden, 1836. 
Etym.—Dedicated to Dreyssens, a Belgian physician. 
Syn.—Mytilina and Mytilomya, Cantr., 1847. Congeria, 
Partsch, 1836. Tichogonia, Rossm., 1835. Enocephalus, Mun- 
ster, 1831. 
Distr.—15 sp. Europe, America, Africa. Fossil, 13 sp. 
Hocene—; Britain, Germany. 
Shell like Mytilus, without its pearly lining; inner layer comw- 
posed of large prismatic cells; umbones terminal; valves 
obtusely keeled; right valve with aslight byssal sinus; anterior 
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