114 GASTEROPODA OF THE INFERIOR OOLITE. 



Oolite is endorsed by Piette and Cossmann. The former figures a variety (op. cit. 

 pi. v, figs. 6 and 7) from the Fuller's Earth of Les Clappes, which is charac- 

 terised by numerous slender longitudinals on each whorl. The same author {pp. 

 cit., p. 46) points out the difference between Al. hamus, Desl., and the fossil so 

 identified by Morris and Lycett. 



Description : 



Length of good-sized specimens . . 25 mm. 



Width of body-whorl to length of shell . . 40 : 100 



Approximate spiral angle . . .32° 



Shell strongly turrited (muricated), fusiform ; apex blunt, but the apical angle 

 only slightly convex, so that the general aspect of the spire is sharp. Number 

 of whorls eleven ; two and a half smooth and convex ; remainder of whorls of 

 spire very angular, the keel being slightly above the middle ; longitudinal costse 

 strong, regular, and numerous, for the most part sloping from right to left. In 

 the majority of specimens from Burton Bradstock these costse are confined to 

 the anterior area of the whorl and fail to reach the suture, but in some cases (fig. 

 6 a) they extend across the keel. There are about five spirals above the keel and 

 usually four below, exclusive of the keel. The body-whorl is bicarinate, with the 

 keels plain, and the spiral ornamentation fainter in most cases ; the posterior keel 

 carries two spinous lumps, one situate half a turn above the wing and the other 

 mid-way between these points. 



The aperture is triangular to trapezoidal. The wing is constituted by the 

 continuation of the posterior or principal keel, and consists of one stout digitation 

 proceeding for some distance at right angles to the axis of the shell and suddenly 

 curving upwards, somewhat after the fashion of a hook (the sharpness of this 

 curve not sufficiently shown in fig. 6 c) ; see also Plate VII, fig. 9. The wing 

 scarcely overlaps the base of the penultimate. The delicate spiral ornamentation is 

 continued on the wing, base, and canal-sheath, being faintly decussated by fine 

 longitudinal lines. Canal broad, moderately long, and curved anteriorly, though 

 not to any great degree. 



Relations and Distribution. — As the type of the Monodactyls Alaria hamus 

 constitutes a standard of comparison both for varieties and species. The Dorset- 

 shire specimens most nearly approach Deslongchamps' fig. 32. In the Normandy 

 specimens which I have seen the wing appears to overlap the base of the penulti- 

 mate somewhat more than in our English examples, and possibly the hook-like 

 curve at the termination of the digitation is sharper, and the digitation itself 

 shorter in the Normandy specimens. The individual figured (PI. VII, fig. 9) is an 

 unusually fine example. 



This species is most abundant in the Parkinsoni-zone of South Dorset, and 

 especially at Burton Bradstock, whence nearly all our best specimens are obtained 



