GASTEROPODA. 15 



Pterocera Bentleyi. Plate III, figs. 15, 15a, var. fig. 16. 



P. Testa turritd, anfractibus convexis, costis transversalibus (4) ; anfractu ultimo per- 

 magno, et costato; labio externo palmetto digitis guinque divaricatis ; canali obliquo elongato. 



Shell turrited, turbinated, whorls convex, costated, costse (4) transverse, last whorl very 

 large and costated, the costse terminate in an expanded palmated wing, digitations five in 

 number, beneath which are numerous diverging lines which connect the wing with the 

 caudal extremity. 



The whorls are oblique in their upper and cylindrical in their lower portions ; their 

 encircling ribs are unequal and irregular ; no other markings are preserved ; but the con- 

 dition of the specimens is scarcely so good as could be wished. The wing is enormously 

 expanded ; the spines extend a little beyond the connecting portions of the wing. 



Locality. The Stonesfield slate at Collyweston has furnished the present specimens. 

 The specific name in compliment to John F. Bentley, Esq., of Stamford, who has enriched 

 our knowledge of the fossils of that locality. 



Alaria. Nov. Gen. 



A. Testa turritd, alatd et caudatd, aid Integra vel digitald, interdum varicem formanti; 

 canali posteriori nullo, labro sinistro tenui, nunquam calloso nee anfractum ultimum obti- 

 genti, labro dextro interdum ultra anfractum ultimum extenso, canali anteriori producto aut 

 breviusculo. 



Shell turrited, winged, and with a caudal extremity, wing entire or digitated, sometimes 

 produced into a thickening or varix, no posterior canal, left lip thin, never thickened, nor 

 extended upon the penultimate whorl, right lip sometimes extended slightly upon the 

 penultimate volution, anterior canal either produced and lengthened or short. 



This genus is constituted to receive a numerous group of winged shells, which are 

 separated from the true Stroinbidse, Rostellariae, and Pterocera? by a simple but important 

 distinctive character, viz. the absence of a posterior channel upon the spire. The greater 

 number of our Great Oolite species of Strombidac will be found to range themselves 

 under this division of the family ; the character of the wing is various, consisting either of 

 a simple, undivided, and thickened process, or divided into two or more digitations ; the 

 channel, likewise, may be either short and straight, or lengthened and curved ; the inner 

 lip is always thin — usually effuse and scarcely visible, but never produced into a thickened 

 posterior ridge, as in the true Rostellariae ; the wing, in some instances, is extended slightly 

 upon the penultimate volution, which is its utmost limit. 



Another character of some importance, first noticed by Mons. Deslongchamps, and 

 which appears to characterise this group of shells, is this : the animal, after having 

 developed the right margin of the shell, continued to increase in growth, and (like the 

 species of Murex and Ranella) reproduced a second dilated and digitated margin, similar 



