﻿50 BEITISH FOSSIL CEPHALOPODA. 



they are more commonly direct in those with longitudinal ornaments, the most 

 oblique being found among the smooth species. The great majority have no curva- 

 ture of their own, but a few show undulations independent of their obliquity. Their 

 distance is utterly variable from species to species, and has a usual tendency to be 

 greater in the young and to become rapidly less at last, though this latter tendency 

 is not so marked as in the Inflati. Their convexity is usually considerable, flat septa 

 being the exception. It is almost always to this genus that detached septa belong. 

 The siphuncle is a very variable organ, both as to position and size, and its variations 

 seem to have no definite relation to those of the other elements. The more common 

 form is the cylindrical, and the more usual position is near the centre. There are 

 undoubted instances in which it is unsymmetrically placed. 



Subdivisions. — The earliest subdivision proposed was that of Quenstedt in 1836, 

 in his ' De Notis Nautilearum Primariis,' which, not being founded on any particular 

 organ, still remains one of the most natural. He recognised — 



1. Vaginati — with large lateral siphuncles, now the subgenus Endoceras. 



2. Cochleati — with large nummuloid siphuncles, forming now a great part of the 

 subgenus Actinoceras. 



3. Gigantei — for Huronia. 



4. Regulares — with smooth surfaces. 



5. Lineati — with longitudinal ornaments. 



6. Undulati — with feeble transverse undulations. 



7. Annidati — with stronger transverse ribbing. 



8. Inflati— in which the body-chamber approaches the form seen in a Gom- 

 plioceras. 



There were also included the Graptolites, but these were subsequently withdrawn. 



M'Coy practically attempted a subdivision of the genus by the introduction of 

 new names, and, like Quenstedt, he borrowed his characters from various parts of 

 the shell. 



De Koninck, in his ' Animaux Fossiles,' 1844, divided the Orthocerata of the 

 Carboniferous rocks into the following groups : — 



1. Gracilia— having a long cylindrical form and simple siphuncle = Regulares 

 of Quenstedt. 



2. Conoidea — with a rapid rate of increase. 



3. Elliptica— having an elliptic section ; practically the same as the Loxoceras of 

 M'Coy. 



4. Nummularia — the Cochleati of Quenstedt. 



5. Annulata = the same group of Quenstedt and the Cycloceras of M'Coy. 



6. Lineata) 



7 Inflata J = t ^ ie same g rou P s °f Quenstedt. 



It is thus seen that the naturalness of Quenstedt's grouping is recognised, as the 



