CEPHALOPODA. 



Division b. — Air-chambers occupying the whole cavity 



OF THE SHELL. 



Piloceras, Salter, 1859. 



Etymology, pilos, a cap, and ceras, a horn. 



Type, P. invaginatum, Salter, Pig. 6. 



Shell, broad, conical, snb- cylindrical, or com- 

 pressed, and slightly curved. Siphuncle and septa 

 combined as a series of conical concave septa, which 

 fit into each other sheathwise. 



Distribution, 3 species. Lower Silurian. Scot- 

 land. Canada. -pifr. 6. Diagram 

 ORTHOCERAS. * (after'salter) . 



Sub -genera : — 



1. Gonioceras, Hall. 1847. 



Etymology, gonios, an angle. 



Type, G. anceps. Lower Silurian. United States. 



Shell, having the general form and structure of Orthoceras, 

 flattened with extremely salient angles ; septa sinuous ; section 

 of shell, an extended ellipse with projecting angles; siphuncle 

 ventral. 



2. Endoceras, Hall, see W. M., ii. p. 192. 



3. Tretoceras, Salter, 1858 (Diploceras, Salter, 1856). 



Etymology, tretos, pierced. 



Type, Orthoceras bisiphonatum, Sowerby. Lower Silurian. 

 Wales. 



Shell elongated ; septa pierced by a sub-central beaded 

 siphuncle, and also by a deep lateral cavity continuous with 

 the terminal chamber, and passing down side by side with 

 the siphuncle — the cavity affecting at least seven of the upper- 

 most septa, if not the whole. 



CYRTOCERAS.f 



Sub -genera : — 



1. Onoceras, see "W. M., ii. 193. " The shells of this genus 

 and Cyrtoceras pass gradually into each other, but Onoceras may 

 be retained for those species which are much inflated in the ante- 

 rior half or two-thirds of the shell length " (Billings) ; and 

 " which have a more or less strangulated aperture " (Barrande). 

 * See p. 190. t See p. 194. 



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