348 me. l. f. spath ox the [Dec. 1 9 14, 



coarsely-ornamented Tragophylloeeras ibex. With the exception of 

 Tr. numismale and Tr. elteni, which are quoted by Pompeckj 

 from the Jamesoni Zone, all the forms mentioned above appear 

 to belong to the Ibex Zone. l Pliylloceras'' dolosum Meneghini 

 (1881), included by Pompeckj in the present group, is charac- 

 terized by diphyllic saddles which, together with its occurrence 

 in the Mediterranean province, point to a connexion with JRTiaco- 

 phyUites. * Amaltheus (SpTienodiscus) ' sinister Canavari (1882), 

 which is compared by its author with Tr. Iosco mbi, similarly has a 

 Rhacopliyllites suture. 1 



Before any opinion on the interconnexion of these forms can be 

 expressed, the ontogenetic evidence regarding Tr. ibex and Tr. 

 numismale must be examined. The latter, according to Pom- 

 peckj (1893, p. 16), has constrictions up to the diameter of about 

 20 mm., a section which is as high as it is wide still at a diameter 

 of 4 mm. ; whereas at 7 mm. the relation of height to thickness is 

 as 31 : 25, instead of 31 : 17 as in Tr. loscombi. On the other hand, 

 the suture of Tr. numismale shows a beginning of the division of 

 the saddles at the diameter of 4 mm. only, whereas this stage is 

 reached much earlier in Tr. loscombi. The median saddle in the 

 ventral lobe of Tr. numismale also is much simpler than it is in 

 Tr. loscombi. Prom the study of the ontogeny of Tr. loscombi it 

 is clear that such a stouter- whorl ed form with constrictions per- 

 sisting longer, and with a simpler suture-line, 3 was in the direct 

 line of ancestry of Sowerby's ammonite. Now T Pompeckj (1893, 

 p. 18) considered the developmental series numismale-weclisleri- 

 ibex more probable than the two developments put forward by 

 Putterer, the latter tending to develop densicostate and sparsi- 

 costate forms respectively. Neither author, however, seems to 

 have been aware of the existence of a striate Tragopliylloceras 

 fauna in ipost-ibex times. 



As regards Tr. ibex, constrictions persist, according to Pompeckj, 

 up to the diameter of 5 mm., and the whorls soon become very 

 thin with an almost sharpened periphery. A comparison of text- 

 fig. 1 m (p. 341), showing the suture of Tr. loscombi at the diameter 

 of 20 mm., with that of Tr. ibex at 25 mm. (Pompeckj, 1893, p. 23, 

 text-fig. 3) will also testify to the simpler development in the 

 latter. It is true that fig. 1 b of Pompeckj (1893, p. 23), taken at 

 about the diameter of the accompanying fig. 1 li (p. 341), shows 

 a much greater advance ; but, on the other hand, from the evidence 

 •of }T>ung specimens of Tragopliylloceras ibex from Gloucestershire 



1 In 1888, Canavari (' Contribuzione alia Fauna del Lias inferiore di Spezia ' 

 Mem. R. Com. Geol. Ital. vol. iii, pt. 2, p. 34) assigned this form to the 

 genus Oxyiioticeras ! 



2 Pompeckj bases his distinction between Tr. loscombi and Tr. numismale 

 partly on the presence of four auxiliary lobes only in the former, but six in 

 the latter ; in view of the developmental evidence this need, however, cause 

 no uneasiness. A comparison of the section of Sowerby's type (fig. 1, 

 PI. XLIX) with Quenstedfs (1885) fig. 9 on pi. xxxvii will show the difference 

 in the umbilical region between the two forms which might easily account 

 for the varying number of auxiliary elements in the sutures. 



