550 JUEASSIC AMMONITES FROM JEBEL ZAGHtTAN. [Dec. I913. 



true reeticostate Seguenzicerates, whose generic difference will be 

 admitted readily by most palaeontologists. 



Many of the ammonites in question occur in the Middle Lias 

 of Italy, and a great number of forms are founded on imperfect 

 specimens, the figures of which are of as much value as those of the 

 older authors, who paid so little attention to the course of the radial 

 line. For instance, the true curve of the ornament is very doubtful 

 in A. d'Orbigny's Ammonites normanianus 1 or in ' Harpoceras- 

 antiquum ' Wright, 2 and I question the correctness in the former, 

 because of the unusual character of the curve clearly shown in a 

 tracing ; the latter form, which, I may mention, does not seem to 

 have been rediscovered and is quoted by its author from the jamesoni 

 zone (!), is represented in three widely differing drawings. The 

 type-specimens are not in the British Museum collection. Finally, 

 the grouping under ' IJaipoceras' boscense Keynes of forms of such 

 widely differing appearance as those figured by Meneghini, Zittel, 

 Geyer, and others, has caused a great deal of confusion. 



The radial curve, then, enables us to separate the Middle Liassic 

 Hildoceratidae into Rectiradiata and Flexiradiata. The former 

 belong to the genus Seguenziceras Levi (=Arieticeras Seguenza) ; 

 but the definition given by Zittel 3 will have to be altered, so as to^ 

 include also forms with a narrow umbilicus and a more compressed 

 section, having simply a carinate venter. Some of the very evolute 

 forms are apparently transitional to Lower Liassic Polymorphids. 

 By far the greater number of the ammonites belonging to the 

 genus Seguenziceras, including the type, are subrecticostate ; but 

 there are lateral branches developing truly reeticostate and rursi- 

 radiate forms respectively. 



The flexiradiate forms are here united in the one genus Proto- 

 grammoceras, but belong to two separate divisions: one being: 

 distinguished by increasing peripheral projection, and giving rise to 

 subfalciradiate and falciradiate forms ; the other, where there is 

 reduction in the peripheral projection, develops subanguliradiate 

 and angulirursiradiate forms. They may, then, be arranged as- 

 follows : — 



f Reetiradiata : example bertrandi (Kil.). 



Genus S egiienziceras -{ Subrectiradiata : example algovianum (Opp.). 



L_ Rursiradiata : example retrorsicosta (Opp.). 

 f TSubfalciradiata : 



example antiquum 

 T • • . • J Geyer non Wright. 



Increase m projection. ■{ Falci ?. adiata . 



example celebratum 

 L Fuc. 



f Subanguliradiata : 

 Genus T> v otogr ammo c eras, -j exa » p]e normani . 



I anum Fuc. non 

 Decrease in projection J A^Hmrsiradiata : 



example lavimamnn 

 I Mgh., especially var. 

 i_ retroflexum Fuc. 



1 'Pal. Franchise : Terr. Jurass. — Cephalopodes ' vol. i (1842-49) pi. lxrsviii 

 &p.291. 



* 'Lias Ammonites ' Monogr. Pal. Soc. (1878-86) pi. Ivii, figs. 1-4 & p. 431. 

 3 ' G-rundziige der Palaontologie ' vol. i (1910) p. 487 (under Arieticeras). 



