INTEODrCTION. 
XIX 
The “ Table of the Xautiloidea ” (A, p. xxii) has been prepared 
with the view of showing the groups into which the species 
described in the following pages are subdivided. It will be 
observed that the present volume terminates with the Cyrto- 
ceratidae. 
It seems scarcely worth while to separate the !Nautiloidea into 
the two very unequal divisions proposed by Dr. Paul Fischer \ viz. : 
Prosiphonata {=.Proclioanites^ Hyatt) and Petrosiphonata (= Meta- 
choanites, Hyatt), since the former includes only the two aberrant 
genera CoTwceras^ Bronn^ (= Bathnoceras, Barr. ^), and Xotho- 
ceras, Barr. the first of which is imperfectly known. At present 
there is no representative of either of these genera in the British 
Museum. 
Some difference of opinion prevails as to the systematic value of 
the groups into which the genus Xautilus has been from time to 
time divided. Meek ^ expressed the opinion that some of its 
extinct forms differ so materially from the typical Xautili, that 
they ought to be at least entitled to rank as distinct subgenera.” 
Amongst these he distinguishes the following, giving a careful 
diagnosis of their characters: — yautilus, Linn, (typical); Temno- 
cheilus, M‘Coy ; Trematodisciis, Meek and Worthen ; Discitesy 
M‘Coy; SolenocheiluSy Meek and Worthen; Hercor/Jossay Conrad; 
Pseudonautilus y Meek. M'aagen® takes a similar view to that of 
Meek, saying that “ there is no doubt that the Nautili as well as 
the Ammonites can be divided very properly into several groups, 
which, if properly defined, might very well prove to be of at least 
subgeneric value.” Mojsisovics after remarking upon the diffi- 
culty of separating the Nautili into genera, or genetic groups, 
on account of the simplicity of the curvature of their septa, which 
thus offer no basis for classification, separates those of the Alpine 
^ ‘ Manuel de Conchyliologie,’ fasc. iv. 1882, p. 404. 
’ ‘ Lethaea Geognostica,’ Band i. 1837, p. 98. 
^ Syst. Sil. de la BohSine, vol. ii. pt, i. 1867, p. 74 ; ibid. pt. iii. 1874, 
p. 792. 
* Ibid, pt, iii. 1874, p. 784. This genus must not be confounded with 
Eichwald’s genus of the same name, ‘Lethaea Kossica,’ yoI. i. 1860, p. 1193, 
which proved to be an imperfect specimen of an Endoccras. 
® United States Geol. Surv. of the Territories, 1876, vol. ix. p, 490. 
® Mem. Geol. Surv. India ; Palaeontologia Indica— Salt-Range Fossils — 
Ser. xiii., i. p. 43. 
‘ Die Cephalopoden der Mediterranen Triasprovinz,’ 1882, p. 265, 
h 2 
