﻿64 BRITISH FOSSIL CEPHALOPODA. 



De Koninck (' Terr. Carb. de Belg.') combines parts of these two classifications, 

 making the following groups : — l.Imperfecti; 2. Striati ; 3. Radiati ; 4. Lcevigati ; 

 5. Clymenios simplices; 6. Clymenice angulatce ; — the definitions remaining as above. 



Setting aside those species which may be conveniently formed into subgenera, 

 the following divisions are suggested as most useful : — 



1. Simplices — of the ordinary form, with or without striated surfaces, including 

 Trocholites. 



2. Radiati — with radiating, sigmoidal, or angular ribs. 



3. Ornati — with ornaments of various kinds, such as tubercles, mostly found in 

 the Carboniferous rocks, equivalent to part of M'Coy's subgenus Temnocheilus. 



4. Sinuosi — with sinuous sutures. 



5. Moniliferi — with a nummuloid siphuncle, found in the Trias only; 

 Subgenera. — 1. Trocholites. — This has the siphuncle within the dorsal lobe; 



but otherwise like one of the Simplices. 



2. Clymenia. — Whorls uncovered, siphuncle within the dorsal lobe, sutures undu- 

 lating or angular. This group, so eminently characteristic of the Trias, is the most 

 worthy of distinction, especially when the sutures are angular. Yet its internal 

 siphuncle may be matched in Trocholites, its sutures in Nautilus sinuatus, and its open 

 whorls in Discites. 



3. Aturia. — This only differs from Clymenia in being involute like an ordinary 

 Nautilus, thus diminishing the distance between the two latter forms. It is confined 

 to Tertiary rocks. 



4. Discites. — In these the whorls are uncovered and usually ornamented in a 

 longitudinal direction. There is a central vacuity, which may have as much as one- 

 third the diameter of the shell. This subgenus constituted the Imperfecti of Quen- 

 stedt, but is quite worthy of a distinct title. It is confined to the Carboniferous rocks. 



5. Nothoceras. — The peculiarities of the siphuncle — which possesses radiating 

 longitudinal lamellae, lies on the convex side of the shell, and is joined to the 

 septa by a neck which turns towards, instead of away from, the mouth — are ample 

 justification for using a distinct title, even if it be not allowed a generic value. 



Range. — The genus Nautilus appears first in the New World and England, eleven 

 species having been recorded from the Lower Silurian of Newfoundland and Canada, 

 and three from our own country, of the subgenus Trocholites. The Upper Silurian 

 species on the contrary, found rarely in England, are most abundant in Bohemia, 

 though few in number even there, including the subgenus Nothoceras. In its typical 

 form it is almost wanting in the Devonian rocks, having but seven representatives 

 at the most; but the subgenus Clymenia is largely developed, and is confined to 

 these rocks. The genus suddenly expands to a maximum in the Carboniferous and 

 develops the subgenus Discites. It almost dies away in the Permian, having only 

 five species, of which three are American. In the Secondary and Tertiary rocks it 



