426 Mr. G. C. Robson on Apovemodon. 



of this interesting form must remain unknown for the present, 

 and that, in consequence, our attempts to allocate a systematic 

 position to it are dependent on the form of a single structure 

 — the radula. 



A full description of the sliell is given below; it is suffi- 

 cient here to remark that it is, in general, a small putelliform 

 shell with a spiral apex like that of CoccaUnaoY Propilidium. 

 It lacks the internal septum characteristic of the latter and 

 the clear-cut sculpture of the former, while its horny texture 

 distinguishes it from both. 



The radula, however, as Dr. H. i\[. Gwatkin pointed out 

 to me, and as I have subsequently verified for myself, 

 resembles that of the Helicid genus Vallonia. In general 

 arrangement the two are in singularly close agreement. The 

 following differences of detail m-iy bs noted and commented 

 upon. In Apuremodon the small central tooth bears a single 

 cusp, though on its expanded base are often seen traces of 

 two lateral cusps. In VaUonia the central tooth, likewise 

 small, is clearly tricuspid, though the lateral cusps are 

 diminutive. In both genera the lateral teeth are in agree- 

 ment, save for the fact that there are five bicuspid teeth in 

 Aporemodon, whereas in Vallonia the last lateral * is usually 

 three-cusped (though not always so), and thus marks a tran- 

 sition-stage between laterals and marginals. In addition, the 

 interior lower angle of the basal plate in Aporemodon is 

 rounded off, reminding us of the condition seen in Siiccinea. 

 The marginal series of Vallonia seems to consist typically of 

 eight teeth ; in Aporemodon there are seven. In general 

 they agree pretty closely, exhibiting each a narrow basal 

 plate, with the outer end tending to exceed theinner in depth, 

 and bearing a number of small cusps and one large one. But 

 whereas in Aporemodon the cusps are four in number and the 

 second is the largest, in Vallonia the cusps are more nume- 

 rous (five or six) and the first cusp (obviously the same as 

 the big cusp of the laterals) is tiie largest. 



In attempting to assign a systematic position to Apore- 

 modon we may at once exclude the shell from consideration, 

 on account of the plasticity of that structure, especially in a 

 case where the simple patelliform shell is in question, and are 

 thus Xthfaate de mieux with the radula. 



It is clear that, for the time being, this genus must be 

 placed near Vallonia. We might be tempted to regard the 

 latter as a representative of a primitive Helicid stock, from 



* Other authors may prefer to regard this tooth as a marginal, but I 

 am inclined to ranlc it with the lateral series on account of its shape and 

 because it occasionally, even in species where it is normall}' three-cusped, 

 exhibits two cusps only. In V. cyclophorella (Sterki, Proc. Acad. N. Sci. 

 Phil. 1893) it is normally bicuspid. 



