L. F. Spath — Jurassic Ammonites from East Africa. 853 



the monophyllic ending of the first lateral saddle of the dorsal suture 

 was characteristic of this group. On examination of the present 

 specimen it is found that these points do not allow of its immediate 

 placing in the series, for, whereas constrictions and umbilicus point 

 to an early form, whorl-shape and suture-line ally the specimen 

 more with the later species of the series. To judge by specimens of 

 S. tortisulcatum, however,^ the secondary forward convexity of the 

 constrictions (on the periphery) becomes more pronounced only 

 in larger examples, and in young specimens there is only the angular 

 bend on the lateral area, whereas the ventral portions of the sulci 

 are straight. Also the section of the inner whorls is much thicker 

 near the umbilical edge and narrower at the venter than it is in the 

 adult whorls. After attaining a diameter of about 7 mm. the pro- 

 portions given in Fig. 56 appear, and the section gradually changes 

 to the quadrangular shape characteristic of the adult S. tortisulcatum, 

 with very square ventral area. 



Although, therefore, in the adult S. tortisulcatum the proportions 

 are different, the inner whorls are comparable to the specimen 

 collected by Professor Gregory, but the constrictions of d'Orbigny's 

 species show a ventral convexity already at a diameter of 9 mm. 

 The course of the constrictions also separates the East African 

 . specimen from S. protortisulcatum Pompeckj sp., found by the 

 writer in profusion in the Argovian of Jebel Zaghuan, Tunisia.^ 

 The whorl-section here is very similar, and the suture-line also 

 agrees very well in general characters, but has a far smaller number 

 of elements. In S. cf. Loryi Munier-Chalmas sp.,^ recorded from the 

 same locality, the smaller umbilicus exactly agrees with the specimen 

 under review, but according to Pervinquiere ^ the typical specimens 

 of this acanthicus- zone ammonite show constrictions only on the 

 body-chamber. This author (after Kilian) also united with S. 

 Loryi the ammonite described by Gemmellaro ^ as Phylloceras 

 silenus Dumortier & Fontannes. The Sicilian form agrees with 

 the specimen here described in dimensions (Gemmellaro, for 

 a specimen of the diameter of 21 mm., gives the percentages as 

 51, 44, and 15) and, to a certain extent, in the suture-line, for in 

 a much larger specimen there are only four monophyllic auxiliary 

 saddles, as in the suture-line of S. protortisulcatmn, given by the 

 writer. ** A number of forms of Sowerbyceras more or less agree with 

 the East African specimen in dimensions and suture-line, but have 

 a different (forward) course of the constrictions, e.g. *S. Tietzei 



1 Some of the specimens examined belong to the Astier Collection, preserved 

 in the British Museum (Nat. Hist.), and d'Orbigny stated that he had received 

 limonitic specimens of A. tortisulcatus from M. Astier (Ter. Cret., p. 164). 



2 Loc. cit., 1913, p. 565. 

 ^ Ibid., p. 566. 



* Mudes de Pal. Tunis., i, Ceph. Ter. Second, 1907, p. 16. 



5 Loc. cit., 1877, p. 185, ph xvi, fig. 1-3 



6 Loc. cit., 1913, pi. liii, fig. 2c. 



VOL. LVII. — NO. VIII. 28 



