MYTILID^. 265 



Shell oval or oblong, thin, moderately inflated, with terminal, or 

 very nearly terminal, incurved beaks, surface finel}^ or radiately 

 striated, somewhat stronger 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' Mod. radiolata (Paris foss., 2d ed., 

 vol. ii, p. 22) is congeneric 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 scar, 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. cinna- 

 mominuH^ Chemn., is a recent representant of Arcopenia, while, 

 on the other hand, cretaceous species, like Ilytilus pileopais^ 

 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, 185T. 



Distr. — 31. pelagica^ Forbes (cxxix, 21). On floating blubber, 

 oflT 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. 



Subfamily DREISSENSi'n^. 



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 ; pallial 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 closel}' related to 

 Mytilus to admit of such a separation. 



Dreissensia, Yan 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. 

 Eocene — ; Britain, Germany. 



Shell like Mytilus, without its pearly lining ; inner layer com- 

 posed of large prismatic cells ; umbones terminal ; valves 

 obtusely keeled ; right valve with a slight byssal sinus; anterior 

 18 



