Yol. 52.] OF THE CHALK ROCK. 87 



forms, in which, in the apical part at any rate, they are flat or 

 concave. 



P. perspectiva is almost the only gasteropod of the Chalk Rock in 

 which the shell is generally preserved. It is also one of the com- 

 monest species ; in the Woodwardian Mnseum there are no less 

 than 65 specimens from a single locality — namely, Cuckhamsley. 



Mantell does not give any exact locality for his Cirrus perspectivus, 

 but simply states ' Upper and Lower Chalk of the South Downs ; 

 rare in the latter deposit.' The forms described by Mantell as 

 Trochus linearis (from the Chalk Marl of Hamsey) and Cirrus 

 depressus (from the Upper Chalk of Lewes) have been considered by 

 d'Orbigny, Morris, and others to be identical with perspectivus : the 

 apparent differences being due merely to conditions of preservation. 

 The examination of a large series of specimens which I have made 

 confirms this view. Geinitz and Fritsch, however, regard P. per- 

 spectiva and P. linearis as distinct species. 



The types of Mantell's Cirrus perspectivus (except fig. 12), C. de- 

 pressus, and Trochus linearis are preserved in the British Museum ; 

 so also are the originals of Sowerby's figures of Cirrus perspectivus 

 and C. depressus, and of Dixon's figure of P. per spectiva. Both 

 Mantell's and Sowerby's specimens are badly preserved, and hence 

 it is very difficult to make out the real characters of the species. 



Affinities. — P. neocomiensis of d'Orbigny, from the Lower Cre- 

 taceous of Trance, is distinguished from P. perspectiva by the 

 smaller spiral angle, the ornamentation consisting of striae instead 

 of ribs, and by the sinus-band being sunk. 



The absence of longitudinal ribs and the presence of strong 

 growth-lines on the base of the shell separate P. plauensis, Geinitz 

 (Cenomanian of Plauen, etc.), from this species. 



P. seriato-granulata, Goldfuss, from the Turonian of Saxony and 

 Bohemia, is near to P. perspectiva, but the shell is more depressed 

 and the base of the whorl is angular. 



Barrois's P. Merceyi, 1 from the zone of Micr. cor-anguinum of 

 Lezennes, etc., is related to P. perspectiva, but the figure given by 

 that author is not sufficiently clear to allow of any detailed com- 

 parison being made. 



Distribution. — England : Chalk Marl of Folkestone and Hamsey ; 

 Lower Chalk of the Isle of Wight; Upper Chalk of Norwich, 

 Northfleet, and Lewes. Chalk Rock of Dover, Winchester, Old- 

 borough Castle, Cuckhamsley, Aston Rowtant, Prince's Risborough, 

 Luton, Hitchin, Underwood Hall near Dullingham, and Carleton 

 Grange. Ireland : Hibernian Greensand. France : Cenomanian of 

 Rouen. Westphalia : Senonian of Haldem, Coesfeld, and Lemforde. 

 Saxony : Planer-Kalk of Strehlen and Weinbohla, rarely in older 

 beds. Silesia : Scaphites-beds of Oppeln. Bohemia : Iser, Teplitz, 

 and Priesen Beds. Bavaria : Regensburg Beds of Galgenburg, 

 Reinhausen, etc. 



1 Ann. Soc. geol. du Nord, vol. vi. (1879) p. 450, pi. xii. f. 2 a, b. 



