124. GASTEROPODA OF THE INFERIOR OOLITE. 
The aperture (fig. 5 a) is widely triangular; the wing is constituted by the 
prolongation of the principal keel, and consists of one long and moderately stout 
digitation, which attains a length equal to the height of the spire before gradually 
curving backwards with a wide sweep towards its terminal point (see Pl. VII, fig. 
10). Delicate lines, in continuation of the spirals, slightly cross-hatched by longi- 
tudinal lines, ornament the wing and digitation, and, in a less degree, the wing and 
canal-sheath. The canal is moderately wide, and extremely long, tapering gradu- 
ally to a point; it has a slight anterior curve. 
Relations and Distribution—It is probable that, in the presence of more perfect 
specimens of Al. Roubaleti and Al. Dundryensis, the differences which now seem to 
separate the var. Dorsetensis from both the other forms might be bridged over. 
At present we must be content to keep them apart, bearing in mind also that, in 
the type specimen of Al. Dundryensis, the processes of whatsoever nature, with the 
exception of the canal-sheath, are altogether wanting. Even granting that Al. 
Dundryensis was possessed of two spines on the keel, of which no mention is made 
in the diagnosis, there still remains a very considerable difference in the ornamen- 
tation of the spire, which is so very plain in the lower whorls of Al. Dorsetensis. 
On the other hand, the spire of Schlumberger’s species has a very great resem- 
blance to the Dorset fossil, but the latter differs in the development of two very 
important curving spines, which in Al. Roubaleti appear as mere spinous swellings. 
Moreover, we really know very little regarding the other processes of Al. 
Roubaleti. 
Owing to the comparative abundance of Al. Dorsetensis, it is possible, from 
one specimen or another, to obtain a tolerably complete notion of the original shell, 
which must be accepted as typical of the group to which it belongs. From the 
unicarinata-group, which shares with it the possession of two similar and similarly 
situated spines, it differs entirely in the character of the spire. From the hamus- 
group it differs, still more widely, in the length and slight curvature of the “tail,” 
and also in the length and moderate curvature of the wing-digitation, as well as in 
the character of the spire (see Pl. VII, figs. 9 and 10, where the two forms are 
well contrasted). 
45. Atarta Rovusateti, Schlumberger: another variety. Plate V, fig. 6. 
Spiral angle 28°. This form is at once distinguished from the numerous speci- 
mens of the far commoner Al. Dorsetensis by the comparative narrowness of the 
spiral angle and the greater delicacy of the ornamentation. A careful examination 
of the figured specimen renders it almost certain that it possessed the two spines 
characteristic of Al. Dorsetensis. 
