﻿56 BEITISH FOSSIL CEPHALOPODA. 



group for these, viz. III. Mediogastric, in which the siphuncle is near the centre. 

 These groups suffice admirably for the Silurian forms, which are all more or less 

 unornamented ; but possibly a fourth group of Ornati should be distinguished, when 

 we include the later Palaeozoic forms. 



Subgenera. — 1. Trigonoceras. — The only peculiarity of this is its triangular 

 section with inflated edges, with which the ornaments are more or less in relation. 

 It appears to be confined to Carboniferous rocks. 



2. Piloceras ? — The remarkable fossils found in the Durness Limestone, and 

 described by Mr. Salter under this title, were considered by him to be complete 

 shells in which the septa and siphuncle were united into conical sheaths, and thus to 

 offer the simplest of Cephalopod forms. The absence, however, of a siphuncle on 

 this interpretation, destroys the only reason for calling the fossil cephalopodous at 

 all ; since many Gastropods, e.g. Euomphalus, have septa-like partitions. At a later 

 date ('Pal. Foss.' 1865), Mr. Billings described some Canadian fossils, in which an 

 undoubted siphuncle of large size, in relation to septa preserved around it, showed, in 

 its interior, conical sheaths like those of Salter's Piloceras, with the exception that 

 they were straight and not so rapidly increasing. As he named these specimens 

 Piloceras, it is evident that he considered the Scotch fossils as siphuncles only, the 

 septa being absent, and this view has been adopted by Barrande. If this be the 

 correct view, these forms might well be placed as a subgenus of Cyrtoceras, charac- 

 terised by the presence of sheaths within the siphuncle, and corresponding to 

 Eadoceras among the Orthocerata. No septa have, however, been ever found 

 associated with them at Durness ; though this is less remarkable, on account of the 

 extreme rarity of septa associated with the siphuncles of Actinoceras at Lake Huron ; 

 on the other hand, the fossils themselves bear a close resemblance to the opercula of 

 the Maclurea, found associated with them. This genus is supposed to be a Gastropod 

 on account of the absence of any hinge between its valves ; but its operculum is 

 singularly like the upper valve of a Radiolite, whose hinge is, to say the least, a 

 peculiar one ; while an allied form from newer rocks, Caprinella, has very similar 

 partitions to those of Piloceras. It is possible, therefore, that Piloceras may form 

 part of a group which should unite Maclurea with the Rudistes, and thus be removed 

 from the Cephalopoda altogether. 



Range. — The genus Cyrtoceras commenced in the Tremadoc rocks, and has 

 abundant representatives throughout the Silurian, Devonian, and Carboniferous, 

 in all fossiliferous localities ; though the maximum of described forms occurs 

 in the Silurian, and there is but one species recorded from Permian rocks in 

 Kansas. Of the subgenera, Piloceras is of Lower Silurian, and Trigonoceras of 

 Carboniferous age. 



