VIII 



RADULA OF PULMONATA 



231 



Pulmonata. — The radula of the Testacellidae, or carnivo- 

 rous land Mollusca, is large, and consists of strong sickle- 

 shaped teeth with very sharp points, arranged in rows with or 

 without a central tooth, in such a way that the largest teeth are 

 often on the outside, and the smallest on the inside of the row 

 (as in Rhytida^ Fig. 139). The number and size of the teeth 

 vary. In Testaeella and Grlandina, they are numerous, consist- 

 ing of from 30 to 70 in a row, with about 50 rows, the size 

 throughout being fairly uniform. In Aerobe they are exceed- 



FiG. 137. — Portions of the 

 radula of Opisthobranchi- 

 ata, illustrating types (6) 

 and (c) ; A, Scaphande?' lig- 

 narius L. ; A', one of the 

 teeth seen from the other 

 side, X 40 ; B, Lamellidoris 

 hilamellata L., Torbay, x 

 60; C, Hydatina physis L., 

 E, Indies, x 75. 



ingly large, and only eight in a row, the outermost marginal 

 being probably the largest single tooth in the whole of the 

 Mollusca. The central tooth is always obscure, being, when 

 present, simply a weaker form of the weakest lateral ; in genera 

 with only a few teeth in a row it is generally absent altogether. 

 The first family, of jaw-bearing snails, the Selenitidae, is 

 distinctly intermediate'. The possession of a jaw relates it to 

 the main body of Helicidae, but the jaw is not strong, while the 

 teeth are still, Avith the exception of the central, thoroughly 

 Testacellidan. The central tooth is quite rudimentary, but it is 



