Vol. 60.J 



UPPER JURASSIC AMMONITES. 



CO 



Fig 



1. — Suture-line of Peri- 

 sphinctes plicatilis, nat. 



size. 



which to base his conclusions, and a cast of so extraordinary a 

 specimen could hardly fail to be misleading. He ignores Peri- 

 sphinctes varioeostatus (Buckland), but this I believe to be the species 

 which he means by P. biplex, or at least a variety of it. The real 

 biplex, I venture to suggest, should be set aside as a freak. 



Under these circumstances, it seems desirable to refigure and 

 redescribe Sowerby's 'types' P. plicatilis and P. biplecc, also Buck- 

 land's P. varioeostatus, and a specimen of the ammonite which has 

 so long been known in England (but not on the Continent 1 ) as 

 Ammonites biplex; namely, Olcostephanus Pallasiaaus (d'Orb.). 



The synonymy given here does not profess to be complete. 



Peris phinctes plicatilis (Sow.). (PI. IX, figs. 1 & 2 & text-fig. 1.) 

 [The ' type '-specimen.] 



: 1818. Ammonites plicatilis, Sow. 'Mineral Conchology ' vol. ii, pi. clxvi. 

 1880. Do. do. do. Damon, 'Geology of Weymouth ' Suppl. 2nd ed. 



pi. xvii, fig:. 3. 



Description. — The cast only is preserved. It is discoidal and 

 compressed. The sides of the whorls are flattened ; the back 



rounded : the cross-section is really 

 oblong, but it has a squarish ap- 

 pearance (PI. IX, fig. 2) owing to 

 the weathering having followed the 

 backward slope of the suspensive 

 lobe. There are seventy fine ribs on 

 the last whorl ; they are directed 

 slightly forward, and fork as they 

 pass over on to the back ; occasion- 

 ally they trifurcate, and still more 

 rarely they remain simple. The 

 back of the specimen is so much 

 worn that the ribs appear to have 

 been interrupted, and in places the 

 siphuncle even is exposed. There 

 are very faint indications of about 

 twelve constrictions, but they are so faint that I should scarcely 

 have noticed them had they not been more distinct in another 

 specimen, which differs from "the 'type' in having (1) one auxiliary 

 lobe less, (2) fifty- four ribs at a diameter of 96 millimetres instead 

 of sixty-eight, and (3) the body-chamber preserved. The last-named 

 occupies nearly four-fifths of the last whorl. 



The suture-line (fig. 1, above) is very complex, the suspensive lobe 

 running back farther than either the first lateral or the siphonal. 



Dimensions : — 



Diameter = 107*2 millimetres; 84 milli- 1 Thickness of the last whorl = 02B8 

 metres. of the diameter. 



Height of the last whorl = 0-321 of the Width of the umbilicus =0-431 of the 

 diameter ; 0359 of the diameter. diameter ; 0'396 of the diameter. 





1 See A. Pavlow, Bull. Soc. Imp. des Nat. Moscou, ser. 2, vol. iii (1880) p. 96; 

 also J. von Siemiradzki, Palaeontographica, vol. xlv (1898) p. 207. 



