Vol. 60.] UPPER JURASSIC AMMONITES. 61 



Dimensions : — 



Diameter =72 millimetres. 

 Height of the last whorl =0302 of the 

 diameter. 





Thickness = 0347 of the diameter. 

 Width of the umbilicus =(M-87 of the 

 diameter. 



Locality and Stratigraphical Position. — The specimen 

 described came from the Kimeridge Clay at Chippinghurst, near 

 Chiselhampton, 6^ miles south of Oxford, and is in the Buckland 

 Collection, in the University Museum, Oxford. The species is the 

 zone-fossil of the Upper Kimeridge Clay. 



Affinities and Differences. — The points of difference be- 

 tween this specimen and d'Orbigny's type have already been referred 

 to. A specimen from Hartwell, in the British Museum (Natural 

 History), forms a link between the two, approaching the former 

 in having three simple ribs, and the latter in having whorls which 

 are not depressed, and only twenty-five ribs at a diameter of 69 milli- 

 metres. As the shell is preserved, the constrictions behind each 

 simple rib are well marked. 



Perispliinctes bipliciformis, Nikitin l is very closely related to it, 

 and so is Ammonites amiulosus, Quenstedt. 2 It may be identical 

 with Am. rotundus, Sow., and if it is so, that name has the priority : 

 I have not adopted it because the 'type ' of rotundus is only a worn 

 fragment, upon which it is quite impossible to found a species. 



In conclusion I have much pleasure in expressing my thanks to 

 Mr. S. S. Buckman and Dr. Henry Woodward for suggestions ; to 

 Dr. A. Smith Woodward for facilities for examining specimens in 

 the Natural History Museum ; and especially to Prof. Sollas for help 

 in every way. 



EXPLANATION OF PLATES IX-XII. 



[All the figures are of the natural size.] 



Plate IX. 



Perisphinctes jplicatilis (Sow.). 

 Fig. 1. Side view. 

 2. Front view. 



This is Sowerby's 'type '-specimen, and is preserve'd in the Buckland Collection, 

 at the University Museum, Oxford. 



Plate X. 



Perisphinctes hiplcx (Sow.). 

 Fig. 1. Side view. 



2. Natural cross-section. 

 This is Sowerby's ' type '-specimen, and is preserved at the British Museum 



(Natural History). 



1 ' Die Jura-Ablagerungen zwischen Rybinsk, Mologa und Myschkin, an der 

 oberen Wolga ' Mem. Acad. Imp. Sci. St. Petersb. ser. 7, vol. xxviii (1881) no. 5, 

 pi. vi, fig. 52. 



2 ' Die Ammoniten des schwabischen Jura ' vol. ii (1886-87) pi. lxxxviii, fig. 22. 



