418 MANUAL OF THE MOLLUSCA. 



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

 umbo. 



Fossil (see Pecten). Lower Silurian — Carb. Spitzbergen — - 

 Australia. 



Gervillia, Defrance. 



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



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



Shell like Avicula; elongated; anterior ear small, posterior 

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

 hinge-teeth obscure, diverging posteriorly. 



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



Suh-gemis ? Bakewellia, King. B. ceratophaga, Schl. Fossil, 

 5 species. Permian, Britain, GeiTnany, Eussia. Shell small, 

 inequivalve, cartilage pits 2 — 5 ; hinge with anterior and pos- 

 terior teeth ; anterior muscular impression and pallial line 

 distinct. 



Perna, Bruguiere. 



Etymology, perna, a shell-fish (resembling a gammon), Pliny. 



Synonyms, Melina, Ketz. Isognomon, Klein. Pedalion, 

 Solander. 



Type, P. ephippium, L. PI. XVII., Fig. 2. 



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

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

 a byssal sinus ; muscular impression double. 



The Pernas vary in form like the Aviculce ; some are very 

 oblique, some very inequivalve, and many fossil species have 

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

 Pernas the pearly layer is an inch thick. 



Vistrihution, 18 species. Tropical seas; West Indies — India 

 — West America. 



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



Suh-genera, Crenatula, Lamk. C. viridis, PI. XYI., Fig. 24. 

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

 pits shallow, crescent-shaped. Distribution, 8 species. North 

 Africa, Red Sea — China ; in sponges. Fossil, 4 species. 



Hypotrema, D'Orb, 18o3. H. rupellensis ( = ? Pulvinites 

 Adansonii, Defrance, 1826) ; Coral-rag, Eochelle. 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, curve^l, with 

 about twelve close -set transverse cartilage grooves. 



