76 CONCHOLOGY. 
2. Genus Perna, Pl. VIII. 
Animal. Body much compressed, the mantle-prolonged pos- 
teriorly in a sort of lobe, and fringed at its inferior edge only; a 
byssus; a single contractile muscle. 
Shell. Irregular, much compressed, subequivalve, form some- 
what variable, gaping at the anterior part of its inferior edge ; 
summit very small: hinge straight, vertical, buccal, toothless ; 
ligament multiple, and inserted in a series of longitudinal and 
parallel furrows ; a subcentral muscular impression. Inhabits the 
Indian seas. Ten species living. Two fossil; one found in 
France and one in Virginia. 
Perna ephippium. Perna sulcata. 
P. isognomon. P. vulsella. 
P. femoralis. P. nuclea. 
P. marsupium. P. avicularis. 
P. obliqua. P. canina. 
8. Genus WMalleus. Pl. VIII. 
Animal. Imperfectly known, but certainly byssiferous, with a 
single retractile muscle. 
Shell. Subnacreous, irregular, subequivalve, inequilateral, 
generally much auriculated anteriorly, and prolonged posteriorly 
into the body, so as to present some resemblance to a hammer; 
summits entirely anterior ; between them and the inferior-auricle, 
an oblique slope for the passage of the bissus; hinge linear, very 
long, buccal, toothless ; ligament simple, triangular, inserted in a 
conical oblique pit, partly external ; a moderately large subcentral 
muscular impression. Inhabits the Australian seas. Six species. 
Malleus normalis. Malleus vulsellatus. 
M. vulgaris. M. anatinus. 
M. albus. M. decurtatus. 
4. Genus Avicula. Pl. IX. 
Animal. Body much compressed; mantle cleft throughout 
its circumference, except along the back, and garnished at its 
free edge with a double row of very short tentacular cirri; foot 
