THE LAMELLIBRANCHIA 263 
Sus-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. 
Famity 1. Lian, 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, Bruguitre ; the individuals of 
this genus form a sort of nest by means of the byssus, or swim by 
Fic. 237. 
A portion of the gill of Venus. e.b.v, efferent branchial vessel ; g.f, gill filaments ; g.v, 
afferent branchial vessels. (After Bonnet.) 
clapping the valves of the shell together. Limaea, Broun. Faminy 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. 
Faminy 3. Exntemipas, Gill. Shell thick, inequilateral, the anterior side 
being the shorter. Monomyarian, with the muscular impression on a 
prominent myophorous apophysis. Genus—EKligmus, Deslongchamps ; 
an extinct genus from the Jurassic. Faminy 4. Prynmaz, Meek. 
Shell elongated. Dimyarian, with a very small anterior adductor 
