418 MANUAL OF THE MOLLUSCA. 
simple, sub-central; pedal scar small and deep, beneath the 
umbo. 
Fossil (see Pecten). Lower Silurian — Carb. Spitzbergen — 
Australia. 
GERVILLIA, Defrance. 
Eiymology, dedicated to M. Gervyille, a French naturalist. 
Example, G. anceps, Pl. XVIL., Fig. 1. 
Shell like Avicula; elongated; anterior ear small, posterior 
wing-like; area long and flat, cartilage pits several, wide apart ; 
hinge-teeth obscure, diverging posteriorly. 
Fossil, 87 species. Carb. — Chalk. Europe. 
Sub-genus? Bakewellia, King. B. ceratophaga, Schl. Fossil, 
5 species. Permian, Britain, Germany, Russia. 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, Retz. Isognomon, Klein. Pedalion, 
Solander. 
Type, P. ephippium, L. Pl. XVIL., 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 Avicule ; 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. 
Distribution, 18 species. Tropical seas; West Indies — India 
— West America. 
Fossil, 830 species. Trias —. United States, Chili, Europe. 
Sub-genera, Crenatula, Lamk. OC. viridis, Pl. XVI., Fig. 24. 
Shell thin, oblong, compressed ; byssal sinus obsolete; cartilage 
pits shallow, crescent-shaped. Distribution, 8 species, North 
Africa, Red Sea — China; in sponges. Jossil, 4 species. 
Hypotrema, D’Orb, 1853. H. rupellensis (=? Pulvinites 
Adansonii, Defrance, 1826); Coral-rag, Rochelle. Shell oblong, 
inequivalye; 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, with 
about twelve close-set transverse cartilage grooves. 
