72 CONCHOLOGY. 



4. Genus Avicula. 

 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 cirrhi ; foot 

 small, canaliculated ; a byssus ; mouth surrounded with fringed 

 lips, besides two pair of labial appendages, a large contractile 

 muscle, nearly posterior. 



Shell. Foliaceous or not ; always nacred, subequivalve, of a 

 subregular form, but somewhat variable ; valves oblique, the left 

 one with a little notch, through which the byssus passes ; hinge 

 linear, toothless, or with two small rudimentary teeth ; ligament 

 more or less exterior, placed in a narrow groove ; one very large 

 posterior muscular impression and one very small anterior. Inha- 

 bits the coast of Devonshire. Fourteen species. 



Avicula nudata. Avicula lotaria. 



A. macroptera. A. heteroptera. 



A. semi-sagitta. ' A. crocea. 



A. falcata. A. Atlantica. 



A. Tarentina. A. papilionacea. 



A. squamulosa. A. physoides. 



A. costellata. A. virens. 



5. Genus JSIeleagrina. 

 .inimal. As above. 



Shell. Subequivalve, rounded, squamous, a sinus posteriorly 

 for the passage of the byssus, at which place the left valve is 

 notched and narrow ; hinge linear and destitute of teeth ; liga- 

 ment marginal, elongated, partly exterior, and dilated in the cen- 

 tre. Inhabits the Indian ocean. Two species, of which our first 

 is the pearl-oyster. 



Meleagrina margaritifera. Meleagrina albina. 



FAMILY XVIII. 



Pectinea. Seven genera. 



1 . Genus Pedum. 

 Animal. Unknown, but probably byssiferous. 



