128 GASTEROPODA OF THE INFERIOR OOLITE. 



makes the whorls angular instead of convex (see PI. VI, fig. 1 b, and enlargement, 

 where the successive stages of growth are very well shown). In the penultimate 

 and antepenultimate the anterior area is excavated, and exposes a rim above the 

 suture, prefiguring, as it were, the anterior keel. The body- whorl carries a promi- 

 nent posterior keel in the middle, which gives rise to a stout digitation ; this is 

 connected by a slight palmation of the wing with a very short process, arising from 

 the anterior keel, which has the appearance of an attempt to form an anterior 

 digitation. The termination of the principal digitation is unknown. The spiral 

 lines or striae are continued on the wing and base of the shell, and slightly cross- 

 hatched with axial lines. The posterior keel carries a short curved spine, half a 

 turn above (i. e. opposite) the base of the wing ; the existence of a spine between 

 this one and the wing is not indicated in any of the specimens hitherto available. 



The aperture is triangular to trapezoidal ; the incrustation on the columellar 

 side extended to the edge of the principal keel ; canal rather narrow and nearly 

 straight, so far as has been observed. 



Relations and Distribution. — The specimens on which the above description 

 is based are all from the Inferior Oolite of North Dorset, but the exact horizon is 

 not known. In two of them the anterior " semidigitation " is well preserved. If 

 we were desirous of obtaining any evidence as to the existence of an apparent link 

 between the Monodactyls and the Didactyls these fossils seem to afford us a clue. 

 The character of the spire, however, points to the latter rather than to the former, 

 and the general affinities are perhaps almost as much with the myurus-gvou]) as 

 with the trifida-group (Chenopus Philippi, Sfc). 



Though mainly confined to North Dorset there are some badly preserved 

 specimens of Ataxia from Dundry in the Bristol Museum, which might probably 

 be classed under one or other of the varieties of Al. Doublieri. 



Var. B. (PI. V, fig. 11.) 



A single specimen from the Cadomensis-bed of Oborne presents us with a 

 spire in a very perfect condition but without processes of any kind. The spiral 

 angle is nearly 40°, and very convex towards the apex. The apicals consist 

 of two and a half whorls, perfectly smooth and convex ; next succeeds a whorl 

 convex and with regular longitudinal costse, extending from suture to suture, and 

 fine spiral striae ; the next whorl, though still very convex, developes a slight keel, 

 whilst very fine spiral lines decussate with costae which are more numerous and 

 finer than in the preceding whorl. Remainder of the spire and body-whorl 

 similar to those of Var. A. 



