J. E. GRAY. 563 



before the branchia : half hermaphrodite : opercu- 

 lum none : protectors or shell not symmetrical, flat 

 or slightly convolute, aperture very large, entire. 

 Marine, breathing water. Gen. Umbrella, Pleuro- 

 branchia, Laminaria. 



Order VI. NOTOBRANCHIA. Branchia arborescent, on the 

 right side of the back, under a fold of the mantle : 

 anus and orifice of generation on the right side : 

 half hermaphrodite : operculum none : protectors 

 or shells not symmetrical, more or less convolute ; 

 aperture effused, entire. Marine, breathing water. 

 Gen. Aplysia, Bulla. 



Order VII. SCHISMATOBRANCHIA. Branchia arborescent, 

 two-lobed on the columella side : mantle with a 

 longitudinal slit over the branchia : rectum passing 

 through the two-ventricled heart, and between the 

 lobes of the branchia : hermaphrodite : operculum 

 none : adductor muscle nearly central : shell not 

 symmetrical ; apex spiral ; columella side perforated 

 above. Marine, breathing water. Gen. Haliotis. 



Order VIII. DICRANOBRANCHIA. Branchia arborescent, 

 two-lobed, one lobe on each side : mantle slit or 

 perforated in front over the branchia : rectum pass- 

 ing through the two-ventricled heart, and between 

 the lobes of the branchia : hermaphrodite : oper- 

 culum none : adductor muscle circular, submar- 

 ginal : shell symmetrical, conical, perforated or 

 slit. Marine, breathing water. Gen. Fissurella, 

 Scutus, Diodora, Imarginula. 



Order IX. CYCLOBRANCHIA. Branchia laminar, round the 

 under suface of the mantle just above the foot: 

 anus .and aperture of generation on the right side : 

 tentacula 2 : hermaphrodite : operculum none : ad- 

 ductor muscle circular, submarginal : shell symme- 

 trical, conical, of one piece, imperforated. Marine, 

 breathing water. Gen. Patella. 



Order X. POLYPLACOPHORA. Branchia laminar, round the 

 under surface of the mantle just above the foot : 

 mantle-edge coriaceous : anus terminal : tentacula 

 none : hermaphrodite : operculum none : protec- 

 tors symmetrical, composed of arched shelly plates, 

 implanted in the middle of the back. Marine, 

 breathing water. 



(a.) Plates placed on the back of the mantle. Gymnoplax 

 or Gymnoplacidse, Acanthochitona, Chiton, Lepi- 

 dochitona. 



o o 2 



