97 



NOTE ON THE EADULA OF POM ATI AS ELEGANS (MULL.). 

 By Rev. E. W. Bowell, M.A. 



Read Uth March, 1908. 

 The radula of this well-known species presents a trifid central tooth 

 of a type somewhat similar to that found in many Pulmonates 

 (Cari/chmm, Succinea, Piinctum). Both back and front of the basal 

 plate are furnished with muscular fibres, and these appear to be in 

 connection with the two large chitinous lamellae which are usually 

 (but I believe erroneously) described as ' jaws.' The fibres are of 

 the semi-striated variety. Two other muscular chains connect the 

 bases of the remaining teeth with the lamellae, one chain on each side. 

 The anatomical appearances leave no doubt that the individual rows 

 of teeth are moved while the radula is in action. The bases of all 

 the teeth, except the centrals, are approximated, as shown in Fig. C. 

 The tooth which appears to be the first lateral is really tricuspid, 

 though the inner cusp is usually hidden by the unfolding of the 

 central one. I believe this tooth is homologous with the capituliform 

 tooth of Neritina. Between it and the central there is a fold 

 (marked x in Fig. D) which probably represents the true first 

 lateral. It is easy to confuse this with the basal plate of the 

 apparent first lateral, but by careful selective staining they may 

 be distinguished. The outside tooth — apparent third lateral — is 

 compound, and bears five large and about forty-five small uncini. 

 In their structure and mode of attachment they are like the marginal 

 uncini of Neritina, which are also attached in rows to folds of 

 membrane. These uncini {Pomatias) are represented in Figs. A and B ; 

 it will be observed that each is composed of two separate elements, 

 differing in refractive qualities, like all other radular teeth. The 

 normal position of these booklets would seem to be that shown in 

 Fig. D ; it is only when the radula is forcibly flattened that they are 

 splayed out as in Fig. C, which is included to illustrate the relations 

 of the bases to which muscular fibres are attached. (The divisions of the 

 scale drawn with each figure represent each a distance of ten raicra.) 



In many specimens the points of the first five mai'ginal uncini 

 (teeth on the compound third lateral) will be found broken oflF, and the 

 basal portion which remains presents the appearance of a square tooth. 

 The squared cusps which have been noted on the centrals and laterals 

 of Vivipara have the same origin ; in all perfect specimens the shai'p 

 triangular point is present, though it is readily broken off, and may 

 disappear in the lifetime of the snail. Thus the radulae of Valvata and 

 Vivipara are essentially of the same type. All the cusps in Pomatias 

 are sharp in young individuals. In Neritina the uncini forming the 

 ' fan ' are pectinated according to the system found in the marginals of 

 Limncea and other Pulmonates, with which they are doubtless homologous. 



So far as the radula goes, Pomatias is much nearer to the rhipido- 

 glossate Neritina than to the taenioglossate Littorina. The actual 

 length of the ' ribbon ' is not at all extraordinary ; it bears about the 

 same ratio to the size of the animal as in Neritina, and it needs no 

 microscope to demonstrate how different is the case with Littorina, 



