278 AVICULID^. 



Crenatula, Lamarck, 1802. 



Syn. — Dalacia, Gray, 1848. 



Distr. — 8 sp. N. Africa, Red Sea, China — in sponges. Fossil, 

 4 sp. Jurassic. C. viridis, Lam. (cxxxi, 65). 



Shell thin, oblong, compressed, bj^ssal sinus obsolete ; cartilage- 

 pits shallow, crescent-shaped. 



'*' ? Leproconcha, Giebel, 1856. 



A small, roundish shell, with the lamellar structure of an 

 Ostrea, umbones nearly central, as in Brachiopoda, outer 

 surface tubercular, hinge-area with three to four ligamental (?) 

 grooves. 



The above name has been proposed for a small Triassic shell 

 which Giebel says comes near to Plicatula or Pulvinites. If the 

 hinge-grooves are ligamental, as the}' are supposed to be, the 

 classification of this genus would be near Pulvinites ; but better 

 material must be examined, in order to define the characters of 

 both these problematic genera. 



Pernostrea, Munier-Chalmas, 1864. 



Distr. — 6 sp. Jurassic ; Europe. P. Bachelieri, M.-C. (cxxx,54). 



Shell rounded or oval, solid, more or less tumid, inequivalve, 

 the left valvebeing in adult specimens attached ; structure lamellar, 

 resembling that of Perna ; beaks usually indistinct, hinge-area 

 broad, or with age becomes more or less reduced in extent, with 

 numerous (4-8) transverse ligamental grooves or pits, as in 

 Perna; muscular impression rather small, subcentral, ovatel}'' 

 rounded, in the right valve. This genus forms a connecting link 

 between Perna and Ostrea, differing from the former especially 

 b}'' its sessile habitat, absence of a byssal sinus and strongly exca- 

 vated muscular scar, from the latter by the presence of separate 

 ligamental grooves. Externally Pernosti*ea is barely distinguish- 

 able from Ostrea. 



Inoceramus, Sowerby, 1814. 



Etym. — Is (inos), fibre; Z:;eramos, shell. 



Syn. — Mytiloides, Brongt. 



Distr. — Fossil, *7.5 sp. ? Silurian, Trias — Cretaceous; South 

 America, United States, Europe, Algeria, Thibet. /. concentricus, 

 Sowb. (cxxx, 55). 



Shell inequivalve, ventricose, radiately or concentrically lur- 

 rowed, umbones prominent; hinge-line straight, elongated; car- 

 tilage-pits transverse, numerous, close-set. 



This genus differs from Perna chiefl}^ in form. /. invohitus 

 has the left valve spiral, the right opercular. I. Guvieri attains 

 the length of a yard. Large flat fragments are common both 

 in the chalk and flints, and are often perforated by Cliona. 



