988 PECTINIDA 
strongly gaping anteriorly ; cardinal line oblique. JL. inflata, 
Chemn. (cxxxii, 96). 
AcEsTA, H. and A. Adams, 1855. Shell thin, inequilateral, 
ventricose, a little gaping; surface covered by radiating striz, 
and concentric growth-lines; ligament-pit oblong, lateral. L. 
excavata, Chemn. (exxxii, 97). 
PLAGIOSTOMA, Sow., 1812. Must be reserved for the species of 
the type of the Liassic Pl. gigantea, for which it was originally 
proposed. It is a very well-marked group of fossil, especially 
mesozoic, Lime, of a semiovate or subtriangular shape, with 
nearly smooth or ‘finely radiately striated surface, the striz being 
generally only conspicuous at the sides of the valves, but nearly 
obsolete in the middle; the ears are thick and unequal, the 
anterior being smaller, and the cartilage-pit is oblique and tri- 
angular, generally very deep. L. Cardiiformis, Sowb. (cxxxii, 
98). 
OTENOSTREON, Hichw., 1867. Subequivalve, with strong radi- 
ating ribs, the large anterior mar gin above, or at the side with a 
distinct byssal sinus. Ct. distans, Kichw. Neocomian of Russia. 
This is another well-marked group of generally large and strongly 
ribbed Lime, the shell of which is often irregular, like that of 
some Hinnites s; when adult, it is characterized by the presence 
of a deep insinuation in the anterior ear for the byssus, but in 
young shells this insinuation is hardly more dey eloped than in 
other allied forms, Lima proboscidea of Sowerby, from Jurassic 
deposits, is another species of the subgenus, and there are a few 
other mesozoic forms which may be referred to it. 
Famity PECTINIDA, 
Shell free or adherent, inequivalve, regular or irregular, 
auricled ; internal ligament inserted in a cardinal pit under the 
beaks—it is sometimes externally prolonged, in the adherent 
species, in a notch between the beaks, . 
No siphons; foot small and cylindrical; mantle open, its 
lobes tentaculated. 
Prcren, O. F. Muller. 
Htym.—FPecten, a comb. Scallop. 
Syn.—Argus, Poli. Discites, Schl. Amussium, Muhlfeldt. 
Distr.—200 sp. World-wide; Nova-Zembla—Cape Horn; 
200 fathoms. Fossil, 450 sp. (including Aviculopecten). World- 
wide; Devonian—. PP. purpuratus, Lam. (cxxxiii, 14). P. 
pallium, Linn. (exxxiii, 13). 
Shell suborbicular, regular, resting on the right valve, usually 
ornamented with radiating ribs; beaks approximate, eared ; 
anterior ears most prominent; posterior side a little oblique; 
right valve most convex, with a notch below the front ear; hinge- 
