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 liues, 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. Boubaleti and Al. Bundryensis, the differences which now seem to 

 separate the var. Borsetensis 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. Bundryensis, the processes of whatsoever nature, with the 

 exception of the canal-sheath, are altogether wanting. Even granting that Al. 

 Bundryensis 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. Borsetensis. 

 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. Boubaleti appear as mere spinous swellings. 

 Moreover, we really know very little regarding the other processes of Al. 

 Boubaleti. 



Owing to the comparative abundance of Al. Borsetensis, 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-growp, 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 PI. VII, figs. 9 and 10, where the two forms are 

 well contrasted). 



45. Alaeia Roubaleti, Schlumberger : another variety. Plate V, fig. 6. 



Spiral angle 26°. This form is at once distinguished from the numerous speci- 

 mens of the far commoner Al. Borsetensis 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. Borsetensis. 



