CRYPTOCEEAS IN SILURIAN BOCKS. 265 



based on characters derived immediately from the shell itself. "For 

 the purpose of classification, four characters, of more or less value, 

 are especially available. These comprise: — (1) the character of the 

 aperture ; (2) the form of the septa ; (3) the position and character 

 of the siphuncle ; and (4) the form aud mode of growth of the shell. 



The aperture may be : (a) open ; (6) contracted. The septa : (a) 

 simple ; (b) augular or lobed. The position of the siphuncle : (a) 

 central or sub-central; (b) internal or "ventral;" (c) external or 

 "dorsal." The siphuncle itself: (a) simple ; (b) complicated. The 

 form of the shell : (a) straight or conical ; (b) arched or " horned" 

 in various ways ; (<) discoidal, with or without contiguous volutions ;: 

 and (d) spiral. 



By means of these characters, all the trustworthy genera of the 

 chambered cephalopods may be arranged, conveniently at least, if not 1 -, 

 naturally, in ten sections or families* : as shewn in the following 

 tabular view : — 



1. GtOMphoceratidjE : — Aperture contracted. Gomphoceras (in- 

 cluding Hall's Orthoceras fusiforrne ;) JPhragmoceras ; Oncoceras ;• 

 Lituites ? 



2. Heterosiphonid^e : — Aperture unknown, perhaps contracted. 

 Siphuncle more or less complicated, or otherwise marginal, with, 

 conical orthoceras-like shell. Septa simple or slightly wavy. 

 (See remarks below.) Endoceras ; Cameroceras I " ; Gonioceras ;■ 

 Ormoceras ; Ascoceras. 



3. Natjtilid^e : Aperture open. Septa- simple, Siphuncle cen- 

 tral or sub -central : Orthoceras; Nautilus? Lituites; Hortolus ; 

 Aploceras (including Hall's Cyrtoceras Annulatum ?) Nautilbceras ; 

 Trochoceras. 



4. Trocholitidje : — Aperture open, Septa simple. Siphuncle 

 internal or " ventral." Trocholites. 



5. Cyrtockratid^ : — Aperture open. Septa simple. Siphunclo 

 external or " dorsal" : — Cyrtoceras; Gyroceras ; Cryptoceras. 



6. Clymenidje : — Aperture open. Septa lobed. Siphuncle in- 

 ternal. Clymenia ; Sub-clymenia. 



* Many palaeontologists will, no doubt, think an extended sub-division of this kind very 

 unnecessary, and prefer to group these forms in two, or at the most, in three families ; but 

 in adopting this plan, the characters of the respective families become ill-defined, and the 

 appreciation of transition groups much weakened ; whilst, at the same time, a necessity is 

 occasioned for the creation of sub-families or tribes. A classification which does not shew 

 upon its face a greater distinction between Goniatites, Ceratites, and Ammonites, than 

 between the last named genus and Hamites or Baculites for example, assuredly has no 

 claim to be considered a natural grouping. In the arrangement given in the text, the 

 second family is merely a provisional one, rendered necessary by our still imperfect know- 

 ledge of its included forms.— E. J. C. 

 VOL. II. — 8 



