574 LECTURE XXII. 



Family CyclostomidcB. Shell with an operculum. 

 Genera Cyclostoma, Pupina, Helicina, 



Family AciculidcB. Shell elongated, cylindrical, with a subspiral 

 operculum. 



Genera Acicula, Geomelania. 



B. DKECIA. 



Order Nucleobranchiata. 

 Branchias, when distinct, packed in small compass with the heart 

 in a dorsal mantle cavity, or in a small symmetrical shell ; sometimes 

 wanting together with the shell. Foot rudimental. 



Family Firolidce. A small branchial shell, or none. 

 Genera Firola, Carinariaf Cardiapoda. 



Family Atlantidce. A body-shell. 



Genera Atlanta^ Porcellia, Bellerophon, Cyrtolites. 



Order Tdbulibranchiata. 



Animals inclosed, with their branchiae, in long, curved, or flexuous 

 shelly tubes. 



Genera Vermetus, Dentalium, Magilus. 



Order Cyclobranchiata. 



Branchiae usually a series of lamellae surrounding the body between 

 the foot and mantle. 



Family Patellidce. Shell with the apex turned forwards. 

 Genera Patella, AcmcBa, Gadinia. 



Family Chitonidce. Shell subdivided into eight pieces. 

 Genera Chiton, Chitonellus, Acanthopleura, 



Order Scutibranchiata. 

 Branchiae protected by a simple or shield-shaped shell. 

 Family Halyotidee. Shell with the outer lip notched or per- 

 forated. 



Genera Haliotis, Scissurella, Pleurotomaria. 



Family Fissurellidce. Shell limpet-shaped, perforated at the apex, 

 or notched anteriorly. 



Genera Fissurella, Emarginula, Parmophortis. 



Order Pectinibranchiata. 



Branchiae pectinated or comb-shaped ; in a special cavity at the 

 fore part of the back. Sexes distinct. 



