286 MINERALS AND GEOLOGY 



which probably served as ballast to counteract the extreme buoy- 

 ancy of the shell, (see figure '232). 



The Tetrabranchiate Cephalopoda (thus limited) may be arranged 

 as first indicated by Barrande in two leading groups, named by 

 Fischer, Retrosiphonata and Prosiphonata. In the first, the funnel- 

 shaped sheaths which surround the siphuncle at each septum, point 

 backwards, and the latter forwards (as in most of the Ammonitoidal 

 Cephalopods) ; but in Canada we have no known examples of the 

 Prosiphonata group. Canadian representatives of the Retrosiphonata 

 may be denned as follows : 



A. SEPTA EXTENDING COMPLETELY ACROSS THE SHELL : 



1 . Aperture of shell contracted to a narrow orifice of definite 

 form.* 



Shell, straight Gomphoceras. Silurian to Carboniferous. 

 Shell, curved Phragmoceras. Silurian. 

 Shell, enrolled Lituites. Silurian. 



. Aperture of shell essentially open : 



Shell, straight Orthoceras. Cambrian to Trias. 

 Shell, curved Cyrtoceras. Cambrian to Permian. 

 Shell, enrolled Nautilus. Silurian to Recent. 



B. SKPTA INCOMPLETE, CONFINED TO ONE SIDE OF THE SHELL. 



Representatives very imperfectly known : they form proba- 

 bly a single genus, Ascoceras confined to Silurian strata. 



* A subdivision of this kind has been objected to, first, because an approach towards a 

 contracted aperture is seen occasionally in other genera of the tetrabranchiate series ; and, 

 secondly, because it is exhibited by both straight and curved forms. But in gomphoceras, &c., 

 the contracted aperture assumes a regular, definite shape ; and as it can hardly have been 

 present in the earlier-formed chambers of the shell, but only in the last, abruptly enlarged 

 body -chamber, it evidently indicates a marked change in the condition of the animal. As 

 regards the second objection, surely the definite shape of the aperture is quite as good a clas- 

 sification-character, used subordinately, as the external configuration of the shell. In the 

 recent classification of ZITTEL (Handbuch der Pal.) gomphoceras, because its shell is straight, 

 is placed in the family of the Orthoceratidce, whilst the slightlj-curved Cyrtoceras is placed in 

 a distinct f amity (Cyrtoceratidce) with Phragmoceras. And, in like manner, Trochoceras, 

 Nautilus, <Sec., are formed in to other distinct families, based on the shape of the shell. There 

 can be little doubt, however, that gomphoceras and phragmoceras are more nearly related than 

 gomphoceras and orthoccras ; and, on the other hand, that Cyrtoceras is more closely related 

 to orthoceras than to phragmoceras. Whilst adhering to the outer shape of the shell as a 

 leading-classification character in these tetrabranchiate forms, ZIT EL departs from it entirely 

 in the more or less closely related Ammonitoidea (still represented by Owen and other author- 

 ities as tetrabranchiate), and he thus places the straight Baculites, the hooked Hamites, the 

 spiral Turrilites, <fcc.,in one family with Lytoceras and other completely enrolled nautiloidal 

 fonns. 



