288 PECTINID-iE. 



stroiigl}^ gsiping anteriorly ; cardinal line oblique. L. injlata, 

 Chemn. (cxxxii, 96). 



ACESTA, H. and A. Adams, 1855. Shell tliin, inequilateral, 

 ventricose, a little gaping; surface covered by radiating striae, 

 and concentric growth-lines ; ligament-pit oblong, lateral. L. 

 excavata^ Chemn. (cxxxii, 91). 



PLAGiosTOMA, Sow., 1812. Must be reserved for the species of 

 the type of the Liassic PI. gigantea, for which it was originally 

 proposed. It is a very well-marked group of fossil, especially 

 mesozoic, Limae, of a semiovate or subtriangular shape, with 

 nearly smooth or finely radiatel}^ striated surface, the strise 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). 



CTENOSTREON, Eichw., 1867. Subequivalve, with strong radi- 

 ating ribs, the large anterior margin above, or at the side with a 

 distinct byssal sinus. Ct. distans, Eichw. Keocomian of Russia. 

 This is another well-marked group of generally large and strongly 

 ribbed Limae, the shell of which is often irregular, like that of 

 some Hinnites ; 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 developed 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. 



Family PECTINID^. 



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. 



Pecten, 0. P. Miiller. 



Etym. — Pecten, a comb. Scallop, 



Syn. — Argus, Poll, Discites, Schl, Amussium, Muhlfeldt. 



DiHtr. — 200 sp. World-wide ; Nova-Zembla — Cape Horn ; 

 200 fathoms. Fossil, 450 sp. (including Aviculopecten). World- 

 wide ; Devonian — . P. piirpuratus, Lam. (cxxxiii, 14). P. 

 pallium, Linn, (cxxxiii, 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- 



