562 HISTORY OF SYSTEMS. 



Subclass II. Cryptobranchia. Branchia under the 

 mantle. Breathing water. 

 Order III. Ctenobranchia. Branchia pectinated in a 

 longitudinal series on the inside of the mantle, on 

 the columella side : mantle free in front, mostly 

 extended into a siphon : unisexual : operculum 

 mostly distinct : adductor muscle attached to the 

 columella of the shell : shell not symmetrical, 

 spiral, or involute ; aperture contracted. Marine, 

 rarely fresh water. Breathing water. 

 (a.) Operculum cartilaginous bullated : floating shell down- 

 wards. Gen. Ianthina. 

 (b.) Operculum spiral articulated to the columella. Ncri- 



tina, Navicellus. 

 (c.) Operculum spiral, free. Nerita. Melas. Operculum 

 ovate, nucleus basillary, spire few-whorled. — Tur- 

 bo. Operculum orbicular, nucleus subcentral, spire 

 few-whorled. — Trochus, Valvata, Cerithium. Oper- 

 culum orbicular, nucleus central, spire many- 

 whorled. 

 ((/.) Operculum annular; nucleus subcentral, regular. — 

 Paludina. (Vivipara : Operculum horny, nucleus 

 lateral. Ampularia : Operculum shelly, nucleus 

 lateral. Bithinia : Operculum shelly, nucleus cen- 

 tral.) 

 {e.) Operculum annular; nucleus apicular, irregular. — 



Murex, Voluta, Strombus, Conus. 

 (/•) Operculum none ; shell subinternal. — Cypra?a, Volva. 

 Order IV. Trachelobranchia. Branchia pectinate, in a 

 transverse row round the inside of the mantle over 

 the neck : mantle free in front : unisexual : oper- 

 culum none : adductor muscle attached to the 

 columella, submarginal : shell not symmetrical ; 

 aperture effused. Marine. Breathing water. 

 (a.) Adductor muscle near the columella: shell ear-shaped; 



apex spiral. — Sigaretus, Cryptostoma. 

 (b.) Adductor muscle submarginal, horse-shoe shaped: shell 

 conical; apex incurved. — Velutina, Capulus, Sto- 

 matia. 

 (c.) Adductor muscle marginal: shell depressed; aperture 



with a transverse internal lip. — Crepidula. 

 (d.) Adductor muscle subcentral: shell conical; aperture 

 with an internal lip. — Calyptrsea, Mitrula. 

 Order V. Monopleurobranchia. Branchia lamellar or 

 arborescent on the right side, between the mantle 

 and the foot : anus behind, orifice of generation 



