THE LAMELL1BRANCHIA 



263 



SUB-ORDER 1. OSTRAEACEA. 



Monomyarian Eulamellibranchia, or with a very small anterior 

 adductor muscle. The mantle is open ; the foot rather small ; the 

 branchiae folded ; the shell inequivalve. 



FAMILY 1. LIMIDAE, D'Orbigny. Foot digitiform, with a bysso- 

 genous apparatus. Borders of the mantle provided with long and 

 numerous tentacles. Gills not united with the mantle. Shell pro- 

 vided with auriculae. Genera Lima, Bruguiere ; the individuals of 

 this genus form a sort of nest by means of the byssus, or swim by 



FlO. 237. 



A portion of the gill of Venus, e.b.v, efferent branchial vessel ; g.f, 

 afferent branchial vessels. (After Bonnet.) 



filaments ; g.r, 



clapping the valves of the shell together. Limaea, Bronn. FAMILY 2. 

 OSTREIDAE, Gray. Foot much reduced and devoid of a byssus. Heart 

 generally on the ventral side of the rectum. The gills fused to the 

 mantle. Shell irregular, fixed by the left and larger valve. Genera 

 Ostraea, Linnaeus ; foot absent in the adult ; eatable and cultivated for 

 commerce ; some species, such as the British 0. edulis, are hermaphrodite. 

 FAMILY 3. ELIGMIDAE, Gill. Shell thick, inequilateral, the anterior side 

 being the shorter. Monomyarian, with the muscular impression on a 

 prominent myophorous apophysis. Genus Eligmus, Deslongchamps ; 

 an extinct genus from the Jiirassic. FAMILY 4. PINNIDAE, Meek. 

 Shell elongated. Dimyarian, with a very small anterior adductor 



