536 MOLLUSCA 



DIVISION 2. T-ffiNIOGLOSSA. 



Shell very variable in shape, with or without siphonal canal; 

 radula (Fig. 830) long and narrow, with 1 central, on each side of 

 which are 1 lateral and 2 marginal teeth, making 7 in each row : about 50 

 families, grouped in 2 subdivisions, including most prosobranchs. 



Key to the subdivisions of Tcenioglossa: 



Oi Foot with a broad sole 1. PLATYPODA 



c, Foot fin-shaped ; animals pelagic 2. HETEEOPODA 



SUBDIVISION 1. PLATYPODA. 



Shell well developed ; foot with a broad sole, with which the animals 

 creep: 48 families, marine and in fresh water. 



Key to the families of Platypoda here described : 



a v Aperture rounded, without siphonal canal (Fig. 831, 2). 

 &! Animals marine. 



G! Shell regularly coiled or much flattened. 



d l Shell not long and slender, but more or less globose or flattened. 

 Ci Shell conical and with a distinct spire (Fig. 772,3). 

 /! Umbilicus open (Fig. 831, 1) or more or less closed by a large callus. 



1. NATICIDAE 



/ a Umbilicus closed 3. LITTOBINIDAE 



e, Shell either not spiral or much flattened. 



/! Spiral either absent or very inconspicuous 2. CAPULIDAE 



/, Shell spiral, but discoid 6. SKENEIDAE 



d, Shell elongate with a high spire. 



6j Shell minute, less than 5 mm. long, with 5 or 6 whorls 4. RISSOIDAR 



e, Shell very long, with about 10 whorls 12. TUERITELLIDA.E 



c, Shell with a very irregular spiral 13. VERMETTDAE 



6, Animals in fresh water. 

 Ci Shell usually minute, less than 5 mm. long, and always less than 10 mm. 



d v Shell usually elongate and conical 5. HYDROBIIDAE 



d, Shell usually flattened and discoid 8. VALVATIDAE 



c, Shell not small, usually much more than 12 mm. long; heliciform 

 (Fig. 772). 



di Snout simple ; tentacles short 7. VIVIPABIDAE 



d, Snout divided into 2 long tentacular lobes ; tentacles long. 



9. AMPULLABIIDAE 



o, Aperture elongate, with siphonal canal (Fig. 872,1). 

 &! Canal short (Fig. 854), sometimes consisting only of the narrowed lower 

 edge of the aperture. 



C! Surface ridged or tuberculated 10. CEBITHIIDAE 



c, Surface usually smooth , . . 11. PLEUBOCEBIDAE 



&, Canal long; aperture usually long and narrow (Fig. 859). 

 c x Spire low or hidden ; aperture nearly as long as the shell. 



d l Spire almost or entirely hidden 15. CYPB^IDAE 



d t Spire free and raised. 

 e x Shell thick, with thick reflexed lips. 



A Outer lip with a notch near the lower end 14. STBOMBIDAE 



/, Outer lip without a notch 17. CASSIDIDAE 



e a Shell thin, with a thin lip 16. DOLIIDAE 



c, Spire high ; aperture not half as long as shell 18. TRITONIDAE 



