418 MAKTJAL OF THE MOtLTTSCA. 



simple, sub-central ; pedal scar small and deep, beneatli tlie 

 umbo. 



Fossil (see Peoten). Lower Silurian — Carb. Spitzbergen — 

 Australia. 



GrERViLLiA, Defrance. 



Etymology, dedicated to M. Gerville, a Frencb naturalist. 



Example, G. anceps, PI. XTII., Fig. 1. 



Shell like Avicula ; elongated ; anterior ear small, posterior 

 wing-like ; area long and flat, cartilage pits several, wide apart ; 

 hinge-teetb. obscure, diverging posteriorly. 



Fossil, 37 species. Carb. — Cbalk. Europe. 



Bub-genus ? BakewelUa, King. B. ceratopbaga, Scbl. Fossil^ 

 5 species. Permian, Britain, Germany, Russia. Shell small, 

 inequivalve, cartilage pits 2 — 5 ; binge witb anterior and pos- 

 terior teeth; anterior muscular impression and pallial line 

 distinct. 



pERlsrA, Bruguiere. 



Etymology, perna, a sbell-fisb (resembling a gammon), Pliny. 



Synonyms, Melina, Eetz. Isognomon, Klein. Pedalion, 

 Solander. 



Type, P. epbippium, L. PL XYIL, Pig. 2. 



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

 cartilage pits numerous, elongated, close-set ; rigbt valve with 

 a byssal sinus ; muscular impression double. 



Tbe Pernas vary in form like tbe Aviculce ; some are very 

 oblique, some very inequivalve, and many fossil species have 

 tbe posterior side produced and wing-like. In some Tertiary 

 Pernas tbe pearly layer is an incb tbick. 



JJistrihution, 18 species. Tropical seas; West Indies ■ — India 

 — West America. 



Fossil, 30 species. Trias — . United States, Cbili, Europe. 



Suh-genera, Crenatulo,, Lamk. C. viridis, PL XYL, Fig. 24. 

 Shell tbin, oblong, compressed ; byssal sinus obsolete ; cartilage 

 pits shallow, crescent-sbaped. Distribution, 8 species, Nortb 

 Africa, Eed Sea — Obina ; in sponges. Fossil, 4 species, 



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

 Adansonii, Defrance, 1826) ; Coral-rag, Rocbelle. Shell oblong, 

 inequivalve; rigbt 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, witl 

 about twelve close-set transverse cartilage grooves. 



