Mr. G. C. Robson on Aporcmodon. 427 



wliicli Aporeniodon was given off as a modified form with 

 uncoiled shell, both forms retaining similar dental characters. 

 But I see no reason on anatomical grounds for regarding 

 VaUonia as i)rimitive. It is true that it lacks the mucus- 

 glands characteristic of the Belogona Siphonadenia in which 

 it is placed by Pilsbry *, Hut that does not necessarily 

 imply I hat it never had any. I am of Pilsbry's opinion that 

 it has lost them through degeneration or retrogression. This 

 being so, we are left, in default of further anatomical know- 

 ledge, with no other course open to us but to regard Apore- 

 niodon as a modified form of VaUonia regarded as a normal 

 Helicid. It may be noted that VaUonia is a fairly old genus 

 palajontologically, going back to the Lower Eocene, and that 

 its geographical distribution does not in any way invalidate 

 the genetic relationship here suggested. 



The possibility is not excluded tliat this may be an extreme 

 case of convergence. 1 am reminded of this possibility by 

 the fact that the lielicoid character of transition between the 

 lateral and marginal teeth of the radula is wanting in 

 Aporemodon, the only noticeable transition being in size. 



My best thanks are due to Dr. H. M. Gwatkin, who has 

 made some admirable preparations of the radula and also drew 

 my attention to the similarity between it and that of VaUonia. 



Aporemodon, gen. nov. 



Radula with formula 6-7 : 5 : 1 : 5 : 6-7, resembling 

 that of VaUonia. Siiell minute, patelliform, with coiled apex. 



Aporemodon tomlini, sp. n. 



Radula with median tooth unicuspid, laterals with one 

 large and one small cusp, internal inferior angle of basal 

 plate rounded, marginals quadricuspid, the second cusp larger 

 than the rest. 



tShell minute (2-9 X 2*0 mm., alt. I'l mm.), horny, sub- 

 diaphanous, ovate-oblong, brownisli-yellow, with radial reddish 

 stripes, occasionally bifurcate and seen more clearly anteriorly, 

 sculptured with concentric lines of growth usually hair-like, 

 occasionally larger, crossed by numerous almost imperceptible 

 radial striae, evanescent in many places ; apex anterior, 

 nearly overhanging the margin, coiled towards the left side. 



* Pilsbry (' Manual of Conchology,' ser. ii. vol. ix. pp. xxxri and 237) 

 thus places VaUonia: — 



[Fani. Helicida.'] 



Tribe V. Belogona. 



aa. B, siphonadenia. 



1. Helixj Leptaxis, Szc. 



2. Helicodonta, VaUonia, HeliceUa, &c. 



