Vol. 70.] DEVELOPMENT OF TRAGOPHYLLOCERAS LOSCOMBI. 351 



One of the important features demonstrated in the development 

 of the suture-line of Tragophylloceras loscombi was the persistence, 



up to a diameter of about 14 mm., of a large terminal leaflet on the 

 external saddle, followed by two smaller ones below. According 

 to Pompeckj (1895). this arrangement, which is typical of Mbjs- 

 varites clio (Mojs.) and M. plauorboides (Gaimbel), occurs, if in 

 a different manner, in JPhylloceras heteropkyllum, but is less 

 distinct in Tragophylloceras ibex. Xow. Branco's figure (1879, 

 pi. ix, tig. 1 q) shows that the suture of Phylloceras hetero- 

 phyllum at 3*25 mm., although very similar, is slightly more 

 advanced than that of Tr. loscombi at the same diameter (see 

 tig. 1 e, p. 341); both are foreshadowed in the suture of Mono- 

 phyllites simonyi at 3 mm. (op. cit. pi. viii, fig. Vm), but there 

 is no resemblance whatever to the development of the Mega- 

 phyllites suture, as Pompeckj considers. In fact, the Triassic 

 Megaphyllites insectum Mojs. shows already at the diameter of 

 2'5 mm. the second lateral lobe as complicated as the first one ; 

 whereas, in the Upper Liassic JPhylloceras heterophyllum, it is 

 perfectly entire up to the diameter of 3 "25 mm. The suture of 

 Pli. heterophyllwm shows a diphyllic external saddle and first 

 lateral saddle already at 7 mm., whereas, as we have seen, at twice 

 that diameter the external saddle of Tragophylloceras loscombi has 

 still a large terminal leaflet ; and although in Tr. ibex, as Pompeckj 

 remarks, this character is less distinct, it is certainly shown on 

 specimens exceeding 8 mm. in diameter. 



Moreover, Pompeckj says that in Psiloceras 1 there is never 

 developed on the external saddle a large upper leaflet followed by 

 two small ones below, such as in Mojsvarites. The development 

 of the suture-line of a specimen of Psiloceras aff. erugatwm (Bean- 

 Phillips) from Kobin Hood's Bay, illustrated in fig. 4 of PI. L, 

 shows that this is not the case. At a diameter of 25 mm. the 

 central terminal leaflet is still the largest, and the suture at 7 mm. 

 diameter will show that the external saddle does not by any means 

 develop three equal leaflets straight from an undivided condition. 

 But this Psiloceras suture at the diameter of 7 mm. is almost 

 exactly like that of the accompanying fig. iy*(p. 341) of Trago- 

 phylloceras loscombi at the diameter of 4 mm. : that is. in a 

 post-constricted stage. 2 



The suture may, indeed, be said to remain in the ' Psiloceras '- 

 stage up to a diameter of 8 mm. Xow. Psiloceras is not the 



1 One form generally included in the genus Psiloceras occupies really quite 

 a distinct position. This is A. hagenoioi Dunker, for which the new genus 

 Psilophyllites [type: P. hagenowi (Dunker), : Pakeontographica ; vol. i, 

 1847, pi. xiii, fig. 22] is now proposed. Dunker's fig. 22c is badly drawn: 

 see Quenstedt (1883) p. 21. His fig. 2 of pi. xvii ought to receive a new name. 

 Until it is shown by ontogenetic evidence that the pseudocei-atitic suture of that 

 form is derived by reduction from a psiloceratid one, Psilophyllites cannot 

 even be included in the Psiloceratidae, as a comparison with the development 

 illustrated in fig. 4 of PI. L will show. 



2 Asymmetry of the suture-line, as characteristic of Psiloceratida?. was 

 noticed by Pompeckj in a specimen of Tr. loscombi from Sables, near Bayeux ; 

 but I have not observed it myself in any specimens. 



Q. J. G. S. No. 279. 2 b 



