RADULA OF OPISTHOBRANCPIIATA 



229 



Proneomenia and Lepidomenia have about twenty falciform 

 teeth, much larger at one end of the radula than the other; 

 formula, 0.1.0. 



Opisthobranchiata. — The radula of the Opisthobranchiata 

 is exceedingly variable in shape, size, and number and character 

 of teeth. Not only do allied families differ greatly from one 

 another, but allied genera often possess radulae widely distinct 



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in plan. Thus, among the Polyceridae, G-oniodoris has no cen- 

 tral tooth, one large lateral and one marginal (form. 1.1.0.1.1); 

 Doridunculus the same, with five marginals (form. 5.1.0.1.5); 

 Lamellidoris one each of median, laterals, and marginals 

 (1.1.1.1.1); Idalia, Ancula, and Thecacera the same as G-onio- 

 doris; Crimora several each of laterals and marginals. Even 

 species of the same genus may differ; thus the formula for 

 Aeolis papulosa is 0.1.0, but for Ae. Landshergi 1.1.1; for PJiiline 

 aperta 1.0.1, but for PJiiline pruinosa 6.0.6. 



It must not be forgotten, however, that a simple repetition 

 of the same tooth, whether lateral or marginal, does not nec- 

 essarily constitute an important characteristic, nor does the 

 presence or absence of a central tooth. In most of the cases 

 mentioned above, the difference in the number of laterals and 

 marginals is due to the multiplication of identical forms, while 

 the central tooth, when present, is often a mere plate or narrow 

 block without cusps, whose presence or absence makes little 

 difference to the character of the radula as a whole. 



There appear to be three well-marked types of radula among 

 the Opisthobranchiata. 



(a) Radula with a single strong central tooth, rows few. 



