574 LECTURE XXII. 



Family Cyclostomidce. Shell with an operculum. 

 Genera Cyclostoma, Pupi?ia, Helicina. 



Family Aciculidce. Shell elongated, cylindrical, with a subspiral 

 operculum. 



Genera Acicula, Geomelaiiia. 



B. DKECIA. 



Order Nucleobranchiata. 



Branchiae, 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 FirolidcB. A small branchial shell, or none. 

 Genera Firola, Carinaria, Cardiapoda. 



Family Atlantidce. A body-shell. 



Genera Atlanta, Porcellia, Bellerophon, Cyrtolites. 



Order Tubulibranchiata. 

 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 PatellidcB. Shell with the apex turned forwards. 

 Genera Patella, Acmcea, Gadinia. 



Family Chitonidce. Shell subdivided into eight pieces. 

 Genera Chiton, Chitonellus, Acanthopleura. 



Order Scutibranchiata. 

 Branchiae protected by a simple or shield-shaped shell. 

 Family Halyotidce. 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, Parmophorus. 



Order Pectinibranchlata. 



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

 fore part of the back. Sexes distinct. 



