GASTEROPODA. 21 



the opposite side of the whorl is another distinctive character. The caudal extremity is 

 short and straight. Length, 10 lines; breadth, including digitations, 9 lines. 

 Locality. This species is found in all the shelly beds, but is far from common. 



Alaria trifida, Phil. sp. Plate III, figs. 11, 11«, 113, lie. 



Rostellaria trifida, Phillips. 1835. Geol. of Yorksh., i, t. 5, fig. 4. 



— btspinosa, Phillips. Geol. of Yorksh., i, t. 4, fig. 32. 



— bicarinata, Goldfuss. Petref, t. 170, fig. 1. 



— trifida, Deslong champs. Mem. Soc. Linn. Normandie, vol. vii, pi. 9, 



figs. 28, 29, 30,31. 



— — Desh. Lam. An. sans Vert., 2d Edit., t. 9, p. 665. 



A. " Testa fusiformi,turritd, transverse striata, anfractibus medio carinato-acutis ; ultimo 

 bicarinato, gibbo ; aid didactyld, digitis in estate adultd longissimis, recurvatis ; in juniore 

 modo unico, modo duobus inaqualibus digitis, seu inferiore, seu superiore longiore ; caudd 

 longissimd, recurvatd; aperturd angustatd." (Deslongchamps.) 



Shell fusiform turreted, transversely striated ; whorls acutely carinated about the middle 

 part ; the last whorl has two carinas, the upper of which is most prominent, and has a 

 prominence or spine opposite to the aperture. The wing is digitated ; when full grown 

 the digitations are very long and recurved, the larger being sometimes the upper, and at 

 other times the lower digitation. In the young state it has only one carina and digitation. 

 The canal is very long and recurved, the aperture small. 



Having had the advantage of examining a large number of specimens, comprising every 

 variety both in form and stage of growth, we feel no hesitation in uniting the two species 

 here indicated. The whorls have every degree of angularity, specimens of A. bispinosa 

 having the lower half of each volution simply cylindrical, the carina not projecting beyond 

 it, and the first three or four whorls are smooth and simply convex, scarcely showing any 

 trace of angularity. The extreme of the other variety has the carina not only angulated 

 acutely, but spread out horizontally into a prominent tabular border. 



The encircling striae are equally variable. In some instances the striae are regular and 

 equal, but more frequently they are alternately large and small ; at other times, however, 

 they are altogether irregular and unequal. 



Locality. This species occurs throughout the whole of the Great Oolite near Minchin- 

 hampton ; even the upper beds, when shelly, not unfrequently contain it. Undoubtedly it 

 is the most common example of the genus. In the Calcareous grit; Oxford Clay; Kelloway 

 Rock, near Scarborough, Yorkshire {Phillips). 



M. E. Deslongchamps describes this species as occurring throughout the Jurassic series 

 of Normandy, viz. the Lias, Fontaine-Etoupefour ; Inferior Oolite, Bayeux ; Great Oolite, 

 Ranville ; Oxford Clay, Vaches-Noires ; Kimmeridge Clay, Villerville. 



4 



