VIII RADULA OF PULMONATA 233 



ill which the aculeate form of tooth — so characteristic of the 

 Agnatha — disappears even in the marginals, and is replaced by 

 teeth with a more or less quadrate base ; the laterals, which are 

 always present, are intermediate in form between the central and 

 the marginals, and insensibly pass into the latter. In size and 

 number of cusps the first few laterals resemble the central tooth ; 

 in the extreme marginals the cusps often become irregular or 

 evanescent. As a rule, the teeth are set squarely in the rows, 

 with the exception of the extreme marginals, which tend to slope 

 away on either side. In some Helicidae there is a slight approxi- 

 mation to the Zonitidae in the elongation of the first marginals. 



The above is the type of radula occurring in the great family 

 Helicidae, which includes not only Helix proper, with several 

 thousand species, but also Arion^ Bulimus, ArioUmax, and other 

 genera. The jaw is almost always strongly transversely ribbed. 



In the Orthalicidae (Fig. 140, 0) the teeth of the radula, 

 instead of being in straight rows, slope back at an angle of 

 about 45 degrees from the central tooth. Tlie central and 

 laterals are very similar, with an obtuse cusp on rather a long 

 stem ; the marginals become bicuspid. 



In the Bulimulidae^ which include the important genera 

 Placostylus, Amphidromus, Partula^ Amphibulirmis, and all the 

 groups of South American Bulimulus^ the jaw is very charac- 

 teristic, being thin, arched, and denticulated at the edges, as if 

 formed of numerous narrow folds overlapping one another. The 

 radula is like that of the Helicidae, but the inner cusp of the 

 laterals is usually lengthened and incurved. In Partula the sep- 

 aration between laterals and marginals is very strongly marked. 



The remaining families of Pulmonata must be more briefly 

 described. In the Cylindrellidae there are three distinct types 

 of radula: («) Central tooth a narrow plate, laterals all very 

 curiously incurved with a blunt cusp, no marginals (Fig. 140, 

 D) ; (5) radula long and narrow, central tooth as in (a), two 

 laterals, and about eight small marginals ; (6?) much more heli- 

 cidan in type, central and laterals obtusely unicuspid, marginals 

 quite helicidan. Type (c) is restricted to Central America, 

 types (a) and (5) are West Indian. 



Pupidae: Radula long and narrow; teeth of the helicidan 

 type, centrals and laterals tricuspid on a quadrate base, mar- 

 ginals very small, cusps irregular and evanescent. This type 



