414 Dr. J. E. Gray on some Families of Bivalve Shells. 



Foot small. Anterior adductor muscle very small. Mantle 

 double-edged, inner edge expanded. Gills small, thick, pinnate, 

 united behind. 



Order II. Pholadacea. 



Mantle with two close, more or less elongate siphonal openings 

 under the hinder adductor muscle. Gills two pair, produced 

 into the inhalant or lower siphon. Pedal openmg generally small. 

 Siphons united (except in Pharus). 



Suborder 1. Orthoconch^. Body symmetrical. Valves equal, 

 gaping at each end. Mantle partly exposed. Cartilage ex- 

 ternal or none. Living perpendicularly in holes in rock or 

 sand. 



A. Cartilage none, replaced hy muscles. 



1. Pholadida, Syn. B. M. 1 842, 76 ; P. Z. S. 1847, 187. Foot 

 clavate, truncated, anterior. Gills two on each side. 



B. Cartilage external, marginal. 



2. Gastrochanad(B, Syn. B. M. 1842, 77; P. Z. S. 1847, 188. 

 Foot small, cylindrical, anterior, not byssiferous. Gills two on each 

 side. Shell gaping in front. Cartilage small, weak. Living 

 enclosed in or imbedded in a shell-case. 



3. Saxicavida, Syn. B. M. 1842 79; P. Z. S. 1847, 193. 

 Foot small, cylindrical, inferior, byssiferous. Gills two on each 

 side ; lips small. Siphons moderate. Shell solid. Hinge-teeth 

 rudimentary. Cartilage thick. 1. Saxicava. 2. Cypricardia. 



4. Pholadomyada, P. Z. S. 1847, 194. Foot short, compressed, 

 with a small bifurcate pedal appendage behind. Siphons united. 

 Pedal opening small. Gills two on each side, thick, united behind 

 the body. Shell oblong, gaping at each end. Hinge-teeth rudi- 

 mentary or none. Cartilage exterior, marginal. 



5. Solenida, Syn. B. M. 1842, 77 ; P. Z. S. 1847, 189. Foot 

 very large, clavate or truncated in front, not byssiferous. Pedal 

 opening anterior. Gills two on each side. Shell cylindrical 

 or oblong, gaping at each end. Hinge-teeth distinct. Cartilage 

 very large, on a distinct fulcrum. Living sunk in the sand. 



Suborder 2. HetergconchtE. Body not symmetrical. Valves 

 unequal. Cartilage internal, in a pit ; hinge simple. Pedal 

 opening inferior. Living lying on the side in sand or mud. 



6. Myada, Syn. B. M. 1842, 78 ; P. Z. S. 1847, 190. Siphons 

 elongate, united, covered with a horny periostraca. Gills two on 

 each side, dependent ; lips small. Shell porcellanous. Hinge 

 toothless. Cartilage-pit in a hollow in one and a concavity in 

 the umbonal surface of other valve. 



