WOODWARD : INAUGURAL ADDRESS. 255 



The Stylommatophora, likewise, are primitive in the arrangement 

 of the radula, most of them having a great number of similar teeth to 

 the transverse row. The oldest of them, Pfiramidula and Jaminia, 

 appear, as well known, in the Coal-measures. The determination of the 

 former is due to Pilsbry, than whom we have no greater living authority 

 on helicoids. It was first referred to Zonites^ and subsequently to 

 Archteozomtes, and under these names still masquerades in geological 

 text-books, even the latest, so gyroscopic in their immutability are 

 these works of instruction. Zonites proper, as typified by Z. Algirm, 

 shows no diminution, but in Vitrea there is a considerable reduction 

 in the number of teeth in each transverse row. Of the British 

 species, V. nitidula has 36, the other species from 12 to 15 laterals in 

 each row on either side of the median tooth, whereas in Helix asjjersa 

 there are about 55. 



As regards the character of the individual teeth, Pilsbry ' points out 

 that the multicuspidate form of the primitive pulmonates gives way in 

 the helicoids to the tricuspid type. In many Endodontidse the teeth 

 are all tricuspid, a form usually correlated with small size and strictly 

 terrestrial habits. All modifications in the teeth proceed from the 

 median line of the radula outwards, the outer marginal teeth being 

 the last to be modified. A study of the marginal teeth, or of those of 

 the embryo, therefore gives a clue in many cases to the ancestral 

 condition of a much modified radula. 



The yet more highly specialized of the pulmonates, the Agnatha, 

 typified by the Testacellidse, which date back to the Cretaceous, have 

 likewise a primitive form of radula, in which, however, the individual 

 teeth have become specialized to fit them for their actively carnivorous 

 habits. 



The radula of the Cephalopoda shows successive diminution in the 

 number of teeth, but the gradation does not quite correspond with 

 their taxonomy. Thus Nautilus on either side of the median has, first, 

 two very similar admedians, then two long, pointed teeth, with 

 a vestigeal basal plate between them and another on the outer margin, 

 indicating that the primitive nautiloid radula had six teeth on either 

 side of the central, or a transverse row of thirteen. The remaining 

 members of the Class, with one exception, have only three laterals on 

 either side ; but Loligo, Polyjnis, and Bolitcena have a vestigeal plate 

 on the outer side. The exceptional genus, Gonatus, has only two 

 laterals on either side. Hoyle ^ has noticed that in the Cephalopoda 

 there is a tendency in the corresponding teeth, especially the median, 

 in following rows to vary slightly in a cycle, five or six rows going to 

 each set. 



[Since the above remarks on the radula were written, a most 

 important paper has been published by Miss Igerna E. J. Sollas 

 (Quart. Journ. Micro. Sci., vol. li, pp. 115-136) dealing with the 

 composition and some points in the development of this organ. 



Manual of Conchology, ser. ii, vol. ix, p. siii. 



" Challenger " Keports : Zoology, pt. xliv (1886), p. 54. 



