52 J. Starkie Gardner — On the Gault Aporrha'idee. 



of the fossil, which may be taken as a type of our so-called Pterocera, 

 is most unlike that of the recent Pterocerata: the lip is less dilated 

 and thickened, the columella and aperture are not ridged, and the 

 digitations are not so variable ; the whole shell is much smaller and 

 more delicate. The recent Pterocerata appear to be a modern group, 

 and to be in part the representatives of the Aporrha'idcs of Cretaceous 

 times. Some of the figures given by D'Orbigny of Continental 

 forms approach, however, more nearly to recent types. 



The British Cretaceous Aporrhaidce may be divided, by the forms 

 of the wing and their ornamentation into four groups, as follows : 



Group 1. — Spire short, ovate, longitudinally striated, carinated, 

 lip furnished with three to four long flexuous recurved digitations ; 

 anterior canal resembling digits in form. 



Type : — Aforkhais retusa, J. Sby. Plate III. Figs. 1-6. 



f Rostellaria retusa, J. Sby., 1836. 



J ■ bicarinata, Desh., 1842. 



Synonyms. ... < Pterocerm , D'Orb., 1842. 



I retusa (Forbes), 1845. Sow. sp. 



{^Harpago relusus (Gabb), 1861. Sow. sp. 



Description. — Shell of a delicate shape, broad and ovate ; the spire 

 short, forming an angle of 37-|° ; whorls six, inflated, convex, of 

 which the last is equal in depth to the other five ; each with two 

 keels, the anterior being hidden by the suture, so that the last whorl 

 alone is seen to be bicarinate, the other whorls seeming to have a 

 single prominent ridge at or about the middle. The chief keels 

 are elevated, narrow and subacute, the spaces between them are 

 ornamented by spiral striae, which are extremely variable in 

 number. On the last whorl, from two to seven or eight thread-like 

 lines occupy the space between the posterior keel and the suture, 

 one to three or perhaps four between the keels, and from six to 

 twelve between the anterior keel and the canal ; these last some- 

 times extend over the canal and the dilatation which unites it with 

 the anterior digit. The spire is seen by aid of lens to be finely 

 ribbed, the riblets being more distinct in crossing the keels : from 

 the third whorl to the apex the keel is often undistinguishable, and 

 the riblets so distinct as to cause a reticulated appearance. The 

 ventral side of the shell is encrusted to the summit with a smooth 

 polished enamel, to which encrustation is due the gibbosity of the 

 last whorL 



Each of the two keels is the basis of a very long flexuous digitated 

 process, which is acutely triangular above, and deeply canaliculated 

 ventrally ; the anterior digit being very thick for half its length, . 

 where there occurs a dilated node, which is flattened and triangular 

 on its under-side ; from this point the digitation tapers to, and is 

 finely pointed at its termination. A third, the uppermost process, 

 prolongs and terminates the mouth into a canal posteriorly, and is 

 recurved backward over the spire, extending far beyond it. The 

 anterior canal is very long and slender, being recurved rather 

 abruptly backwards at about two-thirds of its length. The lip in 

 some adult forms (probably very old individuals) is excessively 



