] GEOLOGICAL MEMOIRS. 



the two great Palaeozoic zones, northern and central, and ends by 

 pointing out fluctuations which time has shown in some of the re- 

 sults obtained by Palaeontology. 



Most of the genera which are not included in the author's classi- 

 fication are considered to have been founded upon incomplete obser- 

 vations upon the form and natural development of the characters 

 which constitute the shell of the Palaeozoic Cephalopods. Por in- 

 stance, the position of the siphon, sometimes in the centre, sometimes 

 near one of the sides, and sometimes upon an intermediate point of 

 the ventro-dorsal diameter has been regarded by scientific men as a 

 differential character of high importance ; and uj)on that considera- 

 tion alone not only different types but even supposed distinct families 

 have been founded. The author, on the contrary, from an examina- 

 tion of the family of Nautilidae (Phi^agmoceras, Gomphoceras, OpJii- 

 d'loceras, and Lituites), asserts that the various forms of these dif- 

 ferent types prove that the siphon does not possess that fixity of 

 position which has been supposed, either in species of the same 

 genus or, indeed, in individuals of different ages in the same species. 

 For in those types in which the position of the gastric region of 

 the mollusk is evident the ventral side and the siphon are more 

 or less independent of each other, and their distance may attain 

 not only the half but even three-quarters of the ventro-dorsal 

 diameter of the shell. On applying this principle, several generic 

 denominations introduced into science must disappear. Among them 

 M. Barrande cites the following: — Sycoceras, Pictet, Cryptoceras^ 

 D'Orb., Naiitiloceras, D'Orb., Aploceras, D'Orb., Melia, Fischer, and 

 Cameroceras, Conrad. 



The study of the genera Phragmoceras and Gomphoceras has pro- 

 duced another important fact relative to the significance of the cur- 

 vature of the shells in their medial plane : — 



In jrhragmoceras the majority of the species are curved in such a 

 manner as to present the excurrent tube, or the ventral side of the 

 mollusk, upon the concave side of the shell. Other species, on the 

 contrary, agreeing with the former in their generic characters, have 

 the ventral side, or excurrent tube, upon the convex side. 



By contrast, in Gomphoceras the greatest number of species 

 show the ventral side of the mollusk corresponding to the relatively 

 convex side, while in some the tube is found to the right of the 

 concave side. Thus in Phragmoceras the endogastric species pre- 

 dominate and the exogastric species are rare, while in Gomphoceras 

 the contrary is the case. This consideration lias led the author to 

 think that the same phenomena will manifest itself in other genera, 

 and has induced him to subdivide each genus into two parallel series, 

 according to the endogastric or exogastric curve of the species. The 

 inevitable result of this conclusion is that generic names which have 

 been founded upon this distinction will disappear, such as the type 

 Cyrtocerina, recently established by Mr. Billings. 



The form of the transverse section in the Nautilidae furnishes one 

 of the most apparent characters of the shell. This form, however, 

 U not invariable in each genus, neither is it possible to take each 



