Vol. 69.] JURASSIC AMMONITES FROM JEBEL ZAGHEAN. 567 



acanthicus zone, and he therefore regards the present form as the 

 commonest Tithonian ammonite of Tunis. Its rarity or absence 

 at Sidi Ba Gubrin is consequently very significant. 



Genus Lytoceras Suess. 

 Lytoceras cf. gastaldii Gemmellaro. 



1870. Lytoceras montanum (pars) Gemmellaro, ' Fauna del Calcario a Tere- 

 bratula janitor del Nord di Sicilia ' Giorn. Sci. Nat. Palermo, pt. 1, 

 p. 33 & pi. vi, fig. 1. 



1872. Lytoceras orsinii Gemmellaro, ' Faune Giur. & Liass. della Sicilia' p. 33 



& pi. viii, figs. 2-3. 

 1875. Lytoceras gastaldii Gemmellaro, ibid. p. 114. 

 1898. Lytoceras cf. polyanchomemum (Gemm.) De Riaz, 'Description des 



Ammonites des Couches a Peltoceras transversarium de Trept (Isere) ' 



p. 39 & pi. xvi, fig. 4. 



The dimensions of the specimen which I refer here are : — 



Diameter 37 millimetres. 



Height of the last whorl 38 per cent, of the diameter. 



Thickness of the JaBt whorl 35 per cent, of the diameter. 



Umbilicus 43 per cent, of the diameter. 



The wide umbilicus and hardly overlapping, slowly increasing 

 Avhorls, with section slightly higher than wide, agree well with the 

 type. The cast is smooth, however, and no trace of the original 

 shell-ornament is left. 



Another specimen has a smaller umbilicus (40 per cent, of the 

 diameter only) and possibly more flattened sides, similar to Lytoceras 

 subtile (Opp.) ; but, as the specimen is somewhat worn, specific 

 determination becomes very difficult. Oppel's type comes from the 

 Tithonian of Stramberg ; on the other hand, L. -polyanchomemum 

 Gemm., with which A. de Riaz compares a specimen from the trans- 

 versarius zone of Trept, has been described from the macrocepJialus 

 beds of Sicily. According to Gemmellaro, the Sicilian form has 

 an umbilicus measuring only 38 per cent, of the diameter, and 

 a high elliptical section ; De Riaz's form is more evolute, however, 

 and comes nearer the form to be described next. 



In Lyeoceras orsinii Gemm. the umbilicus is 40 to 43 per cent., 

 as in my two ammonites ; but the better-preserved specimen, at any 

 rate, seems to resemble the Argovian form more than it resembles 

 this acanthicus-zone fossil. It is unfortunate that one cannot 

 express an emphatic opinion on the age of these forms ; but, as I 

 have pointed out before, the fact is very significant that among 

 a large number of forms belonging to the transversarius zone, the 

 apparent exceptions should belong to the little-changing genera 

 Lytoceras and Phylhceras. 



Lytoceras aff. polycyolem Neum. 



1873. M. Neumayr, 'Fauna der Schichten mit Aspidoceras acanthicum' 



Abliandl. K.K. Geol. Keichsanst. vol. v, p. 160 & pi. xxxi, fig. 4. 

 1877. G. G. Gemmellaro, ' Faune Giur. & Liass. della Sicilia ' p. 188 & pi. xvi, 



fig. 5. 

 1907. L. Pervinquiere, ' Fitudes de Paleontologie Tunisienne ; I — Cephalopodes 



des Terrains Secondaires ' p. 17 & pi. i, fig. 4. 



This ammonite is very evolute, the umbilicus measuring, at least, 



2p2 



