J. E. GRAY. 563 



before the branchia : liall licrmaphrodite : 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, BuUa. 



Order VII. Schismatobranciiia. 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 

 j)erforated 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, subniar- 

 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. Polyplacopiiora. 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. 

 («.) Plates placed on the hack of the mantle. — Gynnioplax 

 or Gymnoplacidae, Acanthochitona, Chiton, Lepi- 

 dochitona. 



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