AND ALLIED FORMS. 33 



oi Amtiicola, thongli of smaller s^izo. Tlie dentition of the lin- 

 gual ribbon (Fig. 24) resembles cousidcrubly that of Pomatiojjsis 



Fig. 24. 



Sayana (Amnicola Saijana, Anthony), figured by Troschel in 

 the " Gebiss der Schnecken," tab. viii, fig. 1. The differences 

 are the following : — The central tooth is somewhat broader, and 

 the lateral denticles of its tridenticulate cusp proportionally 

 larger ; while the denticles at the base are directed inward. 

 The cusp of the intermediate tooth is 4-denticulate exactly as 

 in P. Sayana. But the apices of the two lateral teeth differ 

 consideralih' from those of that species, being each 5-denticulate, 

 with the denticles subequal. 



It will be noticed that, among the several prominent marks of 

 distinction between this dentition and that of the Hydrobiina?, 

 that the basal denticles of the rhachidian tooth are placed, in 

 Pomatiopsis, at or near the base. 



The operculum is very nearly like that of Amyiicola. 



To conclude this description I will give an account of the 

 manner in which the stepping mode^ progression of Pomatiopsis 

 is effected. During this motion the foot is so contracted that its 

 two parts are distinct. In 

 what may be called the ^rsi 

 motion, the anterior part 

 being firmly fixed upon the 

 ground, the posterior part 

 is drawn up to it, by a 

 sliding movement ; — in the 

 accompanying cut (Fig. 25) 



' The progression of the Pomatiopsis has been called " looping," like that 

 of Trancatella. But in TruncuteUa only two points of support are used, 

 while in Pomatiopsis there are three. The motion of the latter animal may 

 perhaps with more propriety be called " stepping." 

 June, 1885. 



