﻿DIRHACHIS. 157 



the character of the animal, and that among the Gasteropoda there is so much 

 similarity in the shells of many widely different organisms that it is very hard to 

 settle by analogy the real position of any genus which is represented only by 

 extinct species. 



Order.— PULMONATA, Mr., 1831. 



I. Family. — AuRicULiDiE, Blainville. 



1. Genus. — Dirhachis, 1 gen. nov. 



Shell elongate, elevated, spiral. Whorls convex, ornamented with spiral ridges 

 reticulated by oblique threads. Mouth small, widely ovoid, with continuous lips. 

 Inner lip bearing two large folds or teeth, which appear to be continued within 

 the shell. Outer lip smooth, bevelled, and slightly crenulated within the margin. 

 Shell-structure massive. 



This genus is formed for a single species, which, as pointed out to me by Mr. 

 E. A. Smith, very nearly approaches Plectotrema. It differs from it in having a 

 smooth outer lip, and only two teeth on the inner lip, as well as in some other 

 particulars. 



1. Dirhachis atavus, n. sp. PI. XXV, fig. 15. 



Description. — Shell small, elevated, conical, spiral. Spire large, rather slowly 

 increasing, of three or more convex, very broad, much-exposed volutions. Suture 

 rather wide and deep, irregular. "Whorls regularly and flatly convex, bearing seven 

 or eight sharp elevated spiral ridges, divided by broad shallow grooves, crenulated 

 by more numerous transverse, regular, close threads or growth-lines. Body-whorl 

 small ; in section sloping rather convexly from the suture to the lower part (which 

 is the widest) and there curving round rapidly to form an oblique base ; ornamented 

 by eleven ridges similar to those in the upper whorls, but broken into tubercles by 

 thicker and coarser growth-lines ; the ridge next the suture being the most promi- 

 nent, and those on the lower part being smaller and closer, and gradually vanishing 

 on the base. No umbilicus. Mouth small, elongate, pyriform, pointed above, 

 rounded below. Outer lip dilate, moderately convex, sharp. Peristome slightly 



1 From Si's, twice, and pa\is, a spine or ridge. 



