CEPHALOPODA. 
DIVISION }.—AIR-CHAMBERS OCCUPYING THE WHOLE CAVITY 
OF THE SHELL. 
PILOCERAS, Salter, 1859. 
Etymology, pilos, a cap, and ceras, a horn. 
Type, P. invaginatum, Salter, Fig. 6. 
Shell, broad, conical, sub-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. Fig. 6. Diagram 
ot Piloeerus 
ORTHOCERAS.* (after Salter). 
Sub-genera :— 
1. GoNnIocERAS, 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 salhent angles; septa sinuous; section 
of shell, an extended ellipse with projecting angles; siphuncle 
ventral. 
2. ENDOCERAS, Hall, see W. M., i. 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.t 
Sub-genera :— 
1. OnocrRAS, see W. M., 11. 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 haye a more or less strangulated aperture” (Barrande), 
* See p. 190. T See p. 194. - 
