136 GASTEROPODA OF THE INFERIOR OOLITE. 



Shell fusiform, scarcely turrited, spire convex. Whorls about nine (only seven 

 observed), tumid ; the two sub-apical whorls have longitudinal costas (rarely to be 

 seen for lack of preservation) ; the two succeeding whorls are either smooth or 

 marked with very fine spirals. Up to this point the increase of the shell is very 

 slow. About the seventh whorl a change takes place, the shell begins to develop 

 more rapidly, a sort of keel is formed somewhat above the middle of the whorl, 

 which is spirally striated throughout ; in the penultimate the keel is more pro- 

 nounced ; above the keel the spiral lines are fine and close, below the keel are three 

 primary spirals with very fine intermediate lines. The body- whorl is ventricose, and 

 bicarinate, the upper carina being the most prominent ; the spiral ornamentation 

 is continued throughout, and into the base of the shell ; there are three principal 

 spiral lines between the digitations. The wing rather broad and didactyl, each 

 carina giving rise to a digitation ; the posterior is somewhat the narrower, and 

 sweeps upwards in a curve whose centre is situated near the apex of the shell. The 

 anterior digitation has a tolerably sharp downward curve; it is short, thick, 

 terminating in a sort of blunt point. 



The aperture is trapezoidal, the outer lip projecting, the wing digitations being 

 slightly channelled. The canal-sheath is thick and short, with a sharp upward 

 curve. 



The variety Spinifera differs from the shells previously described thus : — no 

 longitudinal costse have been detected on the sub-apical whorls (but this may arise 

 from the accidents of preservation) ; the general outline is less smooth, whereby 

 the anterior whorls are rendered somewhat more angular ; the anterior digitation 

 is directed more outwards, and with less of an anterior curve. But the chief 

 difference lies in the tendency to develop spines ; the upper carina of the body- 

 whorl carries one or two spines, which are differently placed in different indivi- 

 duals, sometimes a quarter of a turn, sometimes half a turn, above the base of 

 the posterior digitation. In some specimens there seems to have been some on 

 the penultimate. 



Strictly speaking, this form, which is the commoner of the two, should be 

 regarded as more typical than the smooth specimens. 



Relations and Distribution. — Always bearing in mind that this species is a 

 member of the trifida-groxvp, it may be separated from Al. Lorieri by the non- 

 carinate character of the earlier portions of the spire, by the very different shape 

 of the anterior digitation, by the proportional shortness of the highly curved 

 " tail," and by the more pupoid aspect of the whole shell, besides minor points of 

 ornamentation. The var. Spinifera still further differs in the development of 

 spines. 



It has much closer affinities with Maria pupceformis, D'Archiac, a fossil of the 

 Great Oolite of the East of France. Piette (op. cit., p. 85, PI. 13, figs. 1—8) 



