62 FAUNA OF THE CORNBRASH. 



the Leckenby Collection of the Sedgwick Museum. There is, however, a cast from 

 Bedford which corresponds in the shape of the spire, which, if casts can be named 

 at all, should be placed here. But the last whorl here becomes more angular by 

 the development of more conspicuous keels, the posterior of which shows two 

 spinous risings, while the anterior shows one, before the aperture is reached (see 

 PI. VII, fig. 4). 



(/>) Spire with a median keel. 



Alaria tridigitata (Piette). Plate VII, fig 5. 



1855. Pterocera tridigitata, Piette, Bull. Soc. G-eol. France [2] vol. xiii, p. 89, pi. v, figs. 4 — 7. 

 1864. Alaria tridigitata, D'Orbignv, Pal. Franc., Terr. Jurass., vol. iii, p. 76, pi. x, figs. 4, 5. 



Type. — Shell fusiform, with 7 or 8 smooth, keeled whorls ; the last whorl with 

 two keels. Wing formed by two thick digitations keeled outside and furrowed 

 within. The posterior digitation turns towards the summit of the spire, describing 

 a curve and ends in a point ; the other descends first in a straight line, describes 

 two festoons, and then suddenly rises, becoming thin, and forming a curve. Canal 

 long and broad, descending at first in a straight line and then turning suddenly 

 in the form of a hook. The last whorl on the opposite side from the wing tends 

 to become slightly gibbous and has the keels more marked. From the marly 

 limestones representing the Cornbrash at Rumigny (Ardennes). 



Description . — The specimen figured (PI. VII, fig. 5) does not fully show the 

 remarkable bendings of the type ; but if these be interpreted as the final products 

 of growth under favourable circumstances, our shell is just such as might produce 

 them, as it agrees in all the other characters. The central keel is very marked, and 

 the suture lies, as it were, in a deep valley between two parallel elevations. There 

 is no sign of am* finer ornament, in both cases probably a matter of preservation. 

 The two ridges on the last whorl and the corresponding digitations are very thick 

 and there is a deep excavation between them. Posteriorly the wing sends a branch 

 backwards over the whole length of the penultimate whorl. The whole has a 

 characteristic lumpy aspect, the short canal especially so. The only specimen is 

 from Murcot, and is in the author's collection. 



Relations. — Piette does not compare this with any other species, regarding it 

 as very distinct. Nevertheless it has great resemblance to A. jpwpseformis which 

 accompanies the type, but the canal is more stumpy. The last named has also a 

 greater length of wing before the digitations separate and is strongly striated in 

 the spire. It is probably continued into the Oxfordian A. glaucus, D'Orb. 



