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 costz (rarely to be 
seen for lack of preservation); the two succeeding whorls are either smooth or 
marked with very fine spirals. Up to this pot 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 costee 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 hes 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-group, 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. 
Tt has much closer affinities with Alaria pupeformis, D’ Archiac, a fossil of the 
Great Oolite of the East of France. Piette (op. cit., p. 85, Pl. 13, figs. 1—8) 
