﻿BEITISH FOSSIL CEPHALOPODA. 63 



in the living animal ; and Nothoceras, for a single species in which the septum 

 has its " neck " reversed. 



Description. — The shell is discoidal, and the whorls are in contact throughout. 

 There is often, however, a vacuity in the centre, so that until a complete whorl is 

 formed the shell might be taken for a Cyrtoceras or Gyroceras. The whorls usually 

 overlap, and some species are quite involute; but in others, placed in distinct 

 subgenera, the whorls are only just in contact. Most have no ornaments, but the 

 open-whorled ones may have longitudinal bands or tubercles, or finer riblets and 

 strias. The aperture is bounded by a sigmoid curve which is more or less pro- 

 nounced, but which has no processes, except on the dorsal side in a few: hence 

 called Hercoceras. The septa are concave towards the aperture, and the sutures are 

 slightly sigmoid; but in some they are thrown into deep folds, which in certain 

 cases even become angular. There is always a small dorsal lobe, and the neck of 

 the siphuncle is turned away from the aperture, except in the peculiar form called 

 Nothoceras. The septal surfaces usually have considerable convexity in involute 

 species, but in the more open ones there is scarcely any in the transverse direction. 

 The septa are seldom either very remote or very approximate. The siphuncle 

 occupies all positions, from the ventral side to the dorsal, in which latter case it lies 

 within the dorsal lobe, which is made deeper by the coalescence of the neck, the species 

 showing this being classed as distinct subgenera, Clymenia, Aturia, and Trocholites ; 

 the most common position is towards the centre. The elements of the siphuncle are 

 almost always cylindrical. Epidermids are constantly present on the body-chambers 

 of the Carboniferous Nautili, and the septal surface of the recent species is covered 

 with vascular marks, and a normal line is very common. 



Subdivisions. — Many attempts at subdividing this genus into groups have been 

 made. The earliest was that of Quenstedt, who proposed the following (' De Notis 

 Nautilearum Primariis ') : — 



1. Imperfecti — for those with an open centre. 



2. Clymenia^ simplices — Clymenias with simple sutures. 



3. Clymenia? angulosce — Clymenias with angular sutures. 



4. Moniliferi — with nummuloid siphuncles (in the Trias only). 



5. Bisiphites — with a well-marked dorsal lobe. 



6. Simplices — the ordinary forms with simple sutures. 



7. Undulati — with sigmoidal ornaments forming a sinus on the convex side. 



8. Aganides — with sigmoidal sutures, like Aturia. 



The assemblage of forms thus classed includes several subgenera ; but D'Orbigny's 

 subdivisions (' Pal. Franc. Terr. Jur.') refer only to the ordinary Nautili of the 

 Secondary rocks, which he names : — 



1. Striati, if with fine ornaments; 2. Radiati, if with ribs; 3. or Lcevigati, if 

 without ornaments. 



