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



group of " Phjlloceras " tortisulcatum. The greatest thickness of 

 the whorls is ne5,r the umbilical margin, but the sides are almost 

 parallel and the ventral area is evenly convex, not flattened. The 

 ratio of height to thickness is about 7 : 6, as compared with 4 : 3 

 in Sowerbyceras tortisulcatum, according to Pompeckj,^ and with 

 8:7 at most in ;S'. protortisulcatum Pompeckj sp. The umbilicus 

 is comparatively large and deep ; its walls are rounded and not 

 abrupt. There are about four constrictions to the whorl, beginning 

 as conspicuous grooves with a forward bend at the umbilicus, 

 but becoming less distinct on the lateral area. After bending back 

 again they become once more pronounced, passing across the 

 periphery with a sinus that is concave forwards. 



The suture-line consists of very distinctly diphyllic external, 

 first and second lateral saddles, and of monophyllic auxiliary saddles.. 

 This type of suture-line is well illustrated in de Loriol's ^ figure of 

 that of S. tortisulcatum d"Orbigny sp.^ (which he calls a species of 

 the Renggeri Marls), and in that of S. helios Noetling,'' though here 

 the saddles, perhaps, are drawn so as to appear too plump. There 

 are seven auxiliary lobes and saddles, but the inner of these are merely 

 denticulations, though present already at a diameter of 9 mm. 

 This number agrees with that of the auxiliary lobes in the three 

 Callovian species, S. antecedens (Pompeckj),^ ;S. *' transiens 

 (Pompeckj),*' and S. subtortisulcatum (Pompeckj).'' The internal 

 portion of the suture-line consists of six saddles, and it is important 

 to note the monophyllic character of the first lateral saddle (dorsal), 

 which shows the specimen described here to belong to the " Formen- 

 reihe " of Phylloceras tortisulcatum = Soiverhyceras, and not to 

 the group of Ph. ultramontammi, as the backward bend of the con- 

 strictions of the ventral area would suggest. The internal portion 

 of the suture-line shown in Fig. 5c is not drawn from the same part 

 of the specimen as the external half. 



Observations. — Pompeckj,'^ when establishing the ■'"' Formenreihe " 

 of Phjlloceras tortisulcatum, stated that the developmental tendencies 

 in this series were as follows : (a) increase in the size of the umbilicus 

 from the geologically older to the younger forms ; (&) change of 

 whorl-section from elliptical to quadrangular, with the greatest 

 thickness of the whorl moving nearer and nearer to the umbilicus ; 

 (c) increase in the angularity of the constrictions, with a tendency 

 to bend forward a second time on the periphery ; {d) decrease in the 

 complication of the lobe-line. On account of this last character 

 Pompeckj called the series retrogressive, and he also mentioned that 



^ Loc. cit.. Revision, 1893, p. 54. 

 " Loc. cit., 1893, text-fig. 1, p. 5. 

 •^ d'Orbigny's figure {Ter. Jnra'i.^., pi. 189, fig. 3) is probably inaccurate. 



* Der Jura am Hernion, Stuttgart, 1887 pi. ii, fig. 3^. 



* Log. cit., Revision, 1893, p. 47. 

 ' « Ibid., p. 51. 



7 Ibid., p. 52. 



** Loc. cit.. Revision, 1893, pp. 40-5. 



