560 mk. l. p. spath on [Dec. 1913, 



Genus Pekisphincxes Waag. em. Siem. 

 Perisphinctes cf. bientaszi Teiss. (PI. LII, fig. 4.) 



1899. J. von Siemiradzki, 'Monogr. Beschreib. d. Ammonitengattung Peri- 

 spliinctes ' Palseontographica, vol. xlv, p. 302 & pi. xxvi, fig. 49. 



This is the only specimen of Perispliinctes found in the anceps 

 zone of the Poste Optique ; and, although it resembles, in its red 

 marble matrix, the many perisphinctoid forms collected in higher 

 beds on another side of the mountain, it is clearly distinguished 

 by its ventrally rursicostate ornament. The specimen consists of 

 only a poorly-preserved cast, and its dimensions are :— 



Diameter 110 millimetres. 



Height of the last whorl 31 per cent, of the diameter. 



Thickness of the last whorl 27 per cent, of the diameter. 



Umbilicus 46 per cent, of the diameter. 



The specimen is septate throughout, and its diameter would, 

 therefore, be considerably increased by the presence of the body- 

 chamber, which is of importance in view of the affinities of the 

 ammonite to Grossouvria. 



The whorls are compressed : that is, the section is higher than 

 wide, especially towards the end, showing flat sides between a 

 rounded, but distinct, umbilical border and a circular periphery. 

 The whorl is thickest near the umbilical border. 



The umbilical wall itself is almost perpendicular, and the 

 umbilicus moderately deep and wide. The inclusion is rather less 

 than half of the previous whorl. 



The last whorl of the specimen here described is very badly pre- 

 served ; but, at about 70 mm. diameter, the ornament consists of 

 some thirt}-five primary costae which are thickened, slightly prorso- 

 radiate, and virguloid, with the concave side forwards. They begin 

 on the umbilical edge, and reach to the middle of the side, where 

 they bi- or trifurcate, so that on the periphery we can count about 

 110 delicate rursiradiate costae. Some intercalated secondaries also 

 occur. The course of the whole of the radial curve is therefore 

 sigmoidal, and this character is shown also by the two constric- 

 tions on the whorl (at 70 mm. diameter), although these latter 

 are more oblique : that is, more distinctly prorsoradiate. Slight 

 attenuation of the secondaries is shown along the median line of 

 the periphery. Parabolar knobs are not visible. 



The inner whorls, unfortunately, are ver}' badly preserved, but 

 seem to be regularly costate, as in Quenstedt's Ammonites eon- 

 volutus gigas. 1 



The suture-line is similar to that given by Quenstedt 2 for an 

 ammonite which Siemiradzki includes in the synonymy of Peri- 

 sphinctes orion (Opp.), and resembles also that of Grossouvria 

 curvicosta (Opp.) as given by Siemiradzki. 3 



1 ' Amm. d. Schwab. Jura' 1887, pi. lxxxi, fig. 21 & p. 693. 



2 ' Cephalopoden ' 1849, p. 171 & pi. xiii, fig. 6. 



3 ' Monogr. Beschreib. d. Ammonitengattung Perisphinctes ' Palasonto- 

 graphica, vol. xlv (1898) p. 97 & fig. 8. 



