Vol. 60.] UPPER JURASSIC AMMONITES. 59 



necessary, because the suture-line, where fully exposed, is too much 

 simplified by weathering to be reliable. 



A small portion of the body-chamber is preserved. 



Dimensions : — 



Diameter = 183 millimetres ; 215 milli- 

 metres. 



Height of the last whorl = 0-262 of the 

 diameter ; 0*274 of the diameter. 



Thickness of the last whorl = 0"289 of 

 the diameter; 0294 of the diameter. 

 Width of the umbilicus =0*502 of the 

 diameter ; 0*504 of the diameter. 



The measurements of the height and thickness of the last whorl 

 are not very accurate, as the shell is absent in places and the cast 

 is worn. 



Locality and Stratigraphical Position. — Buckland states 

 that this specimen came from the Oxford Clay at Hawnes, 

 4 miles south of Bedford, but I am of opinion that it came from 

 the Ampthill Clay, and for the following reasons: — 



1. It is distinctively Corallian in appearance. 



2. Hawnes is only 3 miles north-east of Ampthill, and is near 

 the edge of the band of colour indicating Lower Greensand on the 

 Geological-Survey maps. Further, the Geological Surveyors 1 say 

 that 



' traced beyond Ampthill the boundary of the Oxford and Kimeridge Clays is 

 largely concealed for some distance by the Cretaceous rocks.' 



3. It is not pyritized, and T. Roberts 2 remarks that in the 

 Ampthill Clay 



' [fossils] are never pyritized, and on this account the clay is easily distin- 

 guished frum the underlying Oxford Clay.' 



Affinities and Differences. — Perispldnctes variocostatus 

 differs from Siemiradzki's interpretation of P. biplex (Sow.) in the 

 following details : — 



1. It has never more than fifty-live to fifty-seven ribs to a 

 whorl, while P. biplex has seventy in middle-sized whorls, 3 but at 

 a diameter of 100 millimetres it has fifty- three as against fifty. 



2. Its innermost whorls are slightly more e volute. 



The dimensions are practically the same, and the change in the 

 character of the ribs occurs in both at a diameter of about 

 200 millimetres. As to the suture-line, Dr. Siemiradzki speaks 

 doubtfully of that given by A. d'Orbigny 4 as belonging to his 

 biplex. It has a shorter auxiliary lobe than variocostatus. 



The dimensions of P. torquatas, Sow., as tabulated by Dr. Siemi- 

 radzki,' for a diameter of 157 millimetres, are exactly those of 

 P. variocostatus at 183 mm. ; but in other respects the two are quite 



1 H. B. Woodward, Mem. Geol. Surv. ■ The Jurassic Hocks of Britain ' vol. v 

 (1895) p. 138. 



a ' The Jurassic Bocks of the Neighbourhood of Cambridge ' 1892, p. 36. 

 [Sedgwick Prize Essay for 1886.] 



3 Siemiradzki, Palajontographica, vol. xlv (1898) p. 266. 



4 Paleontologie Francaise ' Terrains Jurassiques ' vol. i (1849) pi. cxci. 

 6 Palseontographica, vol. xlv (1898) p. 264. 



