CONCHIFEEA. 263 



Syn. Melina, Retz. Isognomon, Klein. Pedalion, Solander. 



Type, P. ephippium, L. PL XVII. fig. 2. 



Shell nearly equivalve, compressed, sub-quadrate; area wide, cartilage 

 pits numerous, elongated, close-set ; right valve with a byssal sinus ; mus- 

 cular impression double. 



The Pernas vary in form like the Avicula ; some are very oblique, some 

 very inequivalve, and many fossil sp. have the posterior side produced and 

 wing-like. In some Tertiary Pernas the pearly layer is an inch thick. 



Distr. 16 sp. Tropical seas ; W. Indies India W. America. 



Fossil, 30 sp. Trias . U. States, Chile, Europe. 



Sub-genera, CrenaMa, Lamk. C. viridis, PI. XVI. fig. 24. Shell 

 thin, oblong,' compressed; byssal sinus obsolete; cartilage pits shallow, 

 crescent-shaped. Distr. 5 sp. N. Africa, Red Sea China ; in sponges. 



Hypotrema, D'Orb. 1853. H. rupellensis (=? Pulvinites Adansonii, 

 Defr. 1826); Coral-rag, Rochelle. Shell oblong, inequivalve ; right valve 

 flat or concave, with a round byssal foramen near the hinge; left valve 

 convex, with a muscular impression near the umbo ; hinge-margin broad, 

 curved, with about 12 close-set transverse cartilage grooves. 



INOCERAMUS, Sowerby (1834). 



Etym. Is (inos) fibre, Keramos shell. 



Ex. I. sulcatus, PI. XVII. fig. 3. Syn. Catillus, Brongn. 



Shell inequivalve, ventricose, radiately or concentrically furrowed, um- 

 bones prominent; hinge-line straight, elongated; cartilage pits transverse, 

 numerous, close-set. 



This genus diifers from Perna chiefly in form. /. involutus has the left 

 valve spiral, the right opercular. I. Cuvieri attains the length of a yard. 

 Large flat fragments are common both in the chalk and flints, and are often 

 perforated by the Cliona. Hemispherical pearls have been found developed 

 Tom their inner surface, and spherical pearls of the same prismatic-cellular 

 structure occur detached, in the chalk. ( Welherell.} The Inocerami of the 

 gault are nacreous. 



Fossil, 40 sp. Lias Chalk. S. America, U.S. Europe, Algeria, Thibet. 



PINNA, L. 



Etym. Pinna, a fin or wing. Type, P. squamosa, PI. XVI. fig. 23. 



Shell equivalve, wedge-shaped ; umbones quite anterior; posterior side 

 truncated and gaping ; ligamental groove linear, elongated ; hinge edentu- 

 lous ; anterior adductor scar apical, posterior sub -central, large, ill-defined ; 

 pedal scar in front of posterior adductor. 



Animal with the mantle margin doubly fringed ; foot elongated, grooved, 

 spinning a powerful byssus, attached by large triple muscles to the centre 

 of each valve; adductors both large; palpi elongated; gills long, 



N 3 



