W. a. Hudleston — Gasteropoda of the Portland Rocks. 387 



clitions prevailed during the deposit of these two sets of calcareous 

 rocks, though perhaps it is nowhei^e so strongly marked as in the 

 Vale of Wardour, where the following may be regarded as the 

 complete sequence between the Purbecks and the Kimmeridge Clay in 

 descending order : — 



1. Upper Cyrena-heAs (Uppermost Portlandian) , wliere all tlie fossils occur as 

 casts, so that specific identification is rendered diflacult. Ceriihium Portland- 

 icum is tlie characteristic univalve. Neritoma sinuosa occurs sparingly. 



2. The chalky series. This is thoroughly marine, and contains the usual large 

 Portlandian fauna, along with some forms which strike one as somewhat 

 peculiar. 



3. The Ct/rena-heds — properly so-called. This group, which is somewhat variable, 

 but usually occurs in three blocks, having a total thickness at the maximum of 

 about eight feet, constitutes the most interesting feature in the Vale of "Wardour. 

 Here the Cyrena type attains its most complete development, and fortunately the 

 shells are in an exceptional state of conservation. Perfect specimens of Cyrena 

 rugosa are to be had in abundance, and with these are associated CeritJiiuin 

 concavum, Sow., occasionally in great numbers, together with many interesting 

 univalves and bivalves, some of which are described in the sequel. 



4. The main building stones. Beds of gritty limestone with much glauconite, 

 containing the usual large Portlandian fauna. There seems to be a considerable 

 break at the top of this series, whilst below it graduates into 



5. Impure, subcalcareous sands and clays, with bands of Trigonice- and other fossils 

 at intervals. 



In Bucks the equivalents of the C^rena-beds are not so well 

 marked, and Cyrena itself is hardly known to me. But there are 

 indications of peculiar conditions which may especially be noted in 

 the uppermost beds of the series, and in portions of the " creamy 

 limestones," where Gasteropoda are more than usually numerous on a 

 certain horizon in particular quarries. 



It is hoped that the above brief explanation may render the 

 geological position of the fossils about to be described pretty clear to 

 those who care to investigate the subject. 



1. PuRPtriioiDEA PORTLANDiCA, sp.n. Plate XI. Figs, la, 16. 

 Drscription. — Specimen from the Portland limestone of Ashendon, 

 Bucks. 



Length 58 millimetres. 



Width 42 



Length of body-whorl to entire shell 70 : 100. 



Spiral angle 78°. 



Shell short, subturrited, angular, scarcely umbilicated. Spire, 

 about five whorls, separated by a suture, which becomes very 

 deep anteriorly. The contour of each whorl is angular : upper 

 margin tabulate and terminating in a salient border or keel from 

 which the sides fall steeply, and even with a certain amount of 

 excavation, towards the lower keel, which, except in the body-whorl 

 and a portion of the penultimate, is hidden by the overlap of the 

 succeeding whorl. These keels were strongly tuberculate ; the 

 tuberculation commencing in the third or fourth whorl, and 

 increasing till in the posterior keel of the body- whorl the orna- 

 mentation was very decided. 



The body-whorl occupies about -Aths of the total height of the 



