CEPHALOPODA. 
265 
Hamilton Group. 
Orthoc-eras crptalum, Hall. Several small fragments preserving the surface characters, which cannot be 
distinguished from this species. 
“ nuntium, “ Fragments resembling this species have been observed from the limestone in 
Ontario, Canada West. 
In addition to the species in the foregoing list there are eight forms known, 
which constitute the characteristic species of the formation. Those described 
in this work are as follows: 
Orthoceras Oliioense, 
“ molestum, 
“ jaculum, 
“ inoptatum, 
“ rudicula, 
“ sirpus, 
“ viator, . 
“ profundum. 
. Hall ' 
. “ > Robust forms (Section a). 
tt 
(6 
• y 
it ) 
n 1 Slender fonns (Section b). 
“ Cone-fluted (Section d). 
Orthoceras molestum. 
PLATE XXXV, FIG. 13. 
Orthoceras molestum, Hall. Illustrations of Devonian Fossils: Cephalopoda. Explanation of plate 35, 
figs. 6, 8. 1876. 
Shell straight, rapidly enlarging from the apex. Transverse section circular. 
Apical angle 9°. Initial extremity unknown. 
Chamber of habitation large, regularly and rapidly enlarging to the aper¬ 
ture, having a diameter at the smaller extremity less than two-thirds of the 
diameter at the aperture. Air-chambers regular, with a depth of nearly 
four mm.; the internal cast of the walls is smooth, and does not depart 
from the general contour of the tube. 
Septa slightly indented by the siphuncle. Sutures distinct, straight and 
horizontal. 
Siphuncle small, excentric, nearer the ventral side. Its other characters 
not observed. 
The test is thin, having a thickness of .3 mm. Surface marked by broad, 
lamellose, subimbricating lines of growth, which make a slight retral curve 
on the ventral side over the siphuncle. 
34 
