CLYDOXICERAS. 53 



Relations. — There are only two already known ammonites that I have been 

 able to find resembling this in any approximate degree — Reineclda rehmarmi, 

 Oppel, and Peris pliinctes mutatus, Trautsehold. The former is figured by Oppel 

 in ' Palaeontologische Mittheilungen,' 1862, pi. 48, fig. 1. The build of the 

 section is of the same type, but there is no proof of the intermission of the ribs 

 on the periphery and therefore of its being a Reinechia, though it is from the 

 " zone of Ammonites macrocephalus." The section and other details of Perisphinctes 

 mutatus are given by Nikitin, Mem. Soc. Imp. Nat. Moscou (1881), vol. xiv, pi. i, 

 figs. 1 — 3. This shows similar side knobs and peripheral ribs, but the shape and 

 proportions of the outer whorl are quite different. It belongs to a slightly higher 

 horizon, but is a companion of P. funatus. 



Perisphinctes, sp. 2. Plate V, fig. 5. 



Description. — A fragment of an outer whorl from the Upper Beds at Holwell 

 may be the indication of another species. It is remarkable for its quadrate 

 section. The umbilical portion of the side of the whorl has large radial swellings, 

 but the periphery is too rough to show the minor ribs ; those of the previous 

 whorl are seen, however, as impressions on the inner side. The specimen might 

 possibly be an indication of P. mutatus. 



Family Harpoceeatidj:. 

 Genus CLYDONICERAS, 1 nov. 



This genus is established on account of the peculiarities of the sutures of 

 A. discus. There are many forms which are discoid, all or almost all of which 

 have been called discus. Thus, v. Bnch, in his ' Trois Planches d'Ammonites,' 

 pi. i, fig. 1, gives a figure under that name which he says is distinguishable from 

 others wrongly so called, by the sutures ; yet the sutures drawn are not those of 

 discus of Sowerby. Similarly the .1. discus of D'Orbigny, ' Pal. Franc. Terr. 

 Jur.,' pi. cxxxi, has sutures quite different from those of Sowerby's species, and 

 more like those of A. aspidoides of Oppel, who first distinguished it from discus 

 by its sutures. The genus may be thus defined : 



Keeled ammonites, with sickle-shaped ornament, whose thickness is small 

 and involution great, thus producing a discoid form. The sutures can scarcely 

 be represented by the usual lobes and saddles, but are thrown into undulations, 

 1 KXvSioviov, a little wave. 



