﻿BEITISH FOSSIL CEPHALOPODA. 51 



only differences are due to the occurrence or absence of such forms as constitute 

 groups not common to the two. 



Other subdivisions have been incidentally proposed, but they are not of sufficient 

 importance to notice, with the exception of that by Barrande. in his classical work 

 on the Silurian Cephalopoda of Bohemia. That author had so much material before 

 him, that he clearly recognised the overlapping of several of the above groups, and 

 in consequence proposed an entirely new one. Marking off Huronia, Endoceras, and 

 Gonioceras as subgenera, he divides the rest into two series : 1. Brevicones, in which 

 the apical angle is 14° or more ; 2. Longicones, in which it is less than that amount, 

 the line being drawn where fewest species lie. The number of species in the former 

 group is too small to admit of any further subdivision, but the latter form no less 

 than sixteen groups, to which names are not assigned, and which are founded on the 

 ornaments of the surface. While the primary subdivision is most useful, the dis- 

 tinctions between the minor groups are in practice too fine to be available, especially 

 when it depends on the direction of the imbrication of transverse lines ; while the 

 distinction of cylindrical and nummuloid siphuncles is not made use of. 



Culling, therefore, from each its excellences, and remembering that all belong 

 confessedly to one genus, and therefore admit of no accurate delimitation, we may 

 group the Orthocerata as follows : — 



Excluding Actinoceras, Endoceras, Tretoceras, Conoeeras, and Gonioceras, the 

 genus consists of — Group I., Brevicones ; Group II., Longicones. The first group 

 are so thoroughly distinct, and at the same time so rare, that any further sub- 

 division is unnecessary, especially as their ornaments are, as a rule, feebly marked. 

 The second group may be classed as : 1. Annulati — having transverse ribs through- 

 out the septal portion, and not confined to the apex. 2. Angulati — having longitu- 

 dinal ribs. 3. Lineati — having fine ornaments, consisting of riblets or strise, longi- 

 tudinal or transverse, or both. 4. Imbricati — with imbricating lamellae turned either 

 towards the aperture or the apex ; and 5. Leves — having no ornaments beyond lines 

 of growth, except near the apex. With this latter group must be temporarily placed 

 all species whose external surface is unknown. 



Subgenera. — 1. Actinoceras. — This group has been generally adopted as a distinct 

 genus, but Barrande rejects the name altogether, on the ground of its being founded 

 upon an error. He considers that the supposed internal tube does not exist, and 

 that both it and the radiating lines are merely intervals between the organic deposits 

 round the necks of the septa, which come into contact, except at the radiating lines. 

 That this is the true cause of the form of the internal structure of the siphon, scarcely 

 admits of a doubt after an examination of the numerous longitudinal sections, show- 

 ing all stages of the deposits, from simple rings to masses occupying the whole 

 siphuncle, which that author gives. Nevertheless there are indications that the 

 internal tube does exist in the form of an inner siphonal membrane. The exterioi 



H 2 



