GWATKIN : SOME MOI.LUSCAN RADUL.^;. I4I 



but they are never uniform, though they approach uniformity in an 

 arboreal type we shall come to presently. In most cases, there is a 

 central tooth, flanked on each side by laterals, and these, again, by 

 marginals. The lateral teeth may be wanting, as in Achatinella, 

 Physa, and what I shall presently call the Southern type of Ancyhis ; 

 and the central tooth is wanting in some of the Agnatha. It is cer- 

 tainly present in some, though not always or in all. Oi Ennea\ for 

 instance, it is not easy to be sure. Its presence does not to myself 

 seem in this case a character of much importance. Central, laterals, 

 and marginals commonly shade into each other, so that while the 

 central tooth is defined by its position, we cannot always draw a clear 

 line between laterals and marginals. Where, however, the distinction 

 is evident, the straight row or gentle curve of the row is likely to be 

 sharply indented, as where we pass from the laterals of Ainylus 

 lacustris to its plate-like marginals. This, however, is more common 

 in the marine genera — Gibbuia, for example, or Turbo. 



Whether the primitive radula consisted of uniform tricuspid teeth, 

 is more than I can say. Direct evidence is wanting, for we have no 

 fossil radulae of Puhnonata, and I do not know that embryology has 

 cleared the question ; but to the best of my knowledge, no such 

 radulse now survive. The teeth are certainly tricuspid over most of 

 the breadth of the radula in some of the Endodoiitidce — a very ancient 

 family — but even there they are not uniform, while other presumably 

 ancient genera, like Laoma, Ainphidoxa, and Ti'acJiycystis, shew no 

 tendency at all to a uniform tricuspid arrangement. However, the 

 central tooth is always symmetrical in the Pulmonata with either one 

 cusp or three. Bicuspid centrals are found only in Carinifex, Plan- 

 orbis, Isidora, Physopsis, and certain species of Ancyhis — whereas 

 Choanomphalns and Erinna are unicuspid, like LivDicea. In Vagiiuda, 

 Physa, Chilina, and Amphibola the central tooth takes peculiar forms 

 not easy to understand. The unsymmetrical centrals seem limited to a 

 few of the Trochidce. As a rule, it is of the same size as the laterals, 

 or a trifle smaller ; but it is much smaller in Achatina and its allies. 

 Much larger I have observed it only in some anomalous American 

 Zonitidce, though Binney so figures it in Aviphibiilima. The laterals, 

 on the other hand, are always unsymmetrical, except perhaps one or 

 two inner laterals in a few of the EndodontidcR. As a rule, the inner 

 cusp disappears almost at once, while the other two persist, growing 

 shorter as we pass away from the centre, and gradually splitting up, 

 the central cusp commonly into two denticles, the outer one into two 

 or more, so that the outer marginals usually have several serrations, 

 in Planorbis and Limnixa a good many. 



This is what we may call the normal radula of the Pulmonata, for 



