488 A. H. Foord—The Genus Actinoceras. 
Actinoceras has been recognized, either in a generic, or in a sub- 
generic sense (under Orthoceras) by Stokes, Portlock, M‘Coy, 
d’Orbigny (as equivalent to Conotubularia, Troost), 8. P. Woodward, 
Hall (under the name of Ormoceras, Stokes), Saemann, Barrande, 
Ferd. Roemer, d’Hichwald, Owen, H. Woodward, Whitfield, Blake, 
and Hyatt. 
The above figures have been drawn from specimens in the British 
Museum, and are designed chiefly to illustrate the structure of the 
siphuncle, in which the characters of the genus reside. 
The shell in some species of Actinoceras (e.g. A. giganteum) is very 
large, in others it is of medium size; it is straight, or slightly bent 
in the young shell, and is circular or subcircular in cross-section. 
The sutures are usually more curved than they are in Orthoceras, 
the necks or recurved portions of the septa are very short, and are 
often incrusted with crystalline deposits ( ‘anneaux obstructeurs ” 
of Barrande), and these sometimes fill the spaces in the siphuncular 
cavity not occupied by the endosiphon and the tubuli proceeding 
from it. The siphuncle is sometimes very large in proportion to the 
shell, it may be equal to half the diameter of the latter; it is 
considerably dilated between the septa, forming thereby a series of 
segments of a compressed-globular shape. The first of these is 
broadly conical, and is perforated just above the apex with a large 
foramen, which served as a passage for the endosiphon from the 
initial chamber into the siphuncular cavity. The shelly wall of the 
segments is rarely preserved, but their calcified linmg membrane 
beneath is very characteristic, presenting a series of longitudinal folds 
or wrinkles. The endosiphon (Fig. 1, en) is provided with a distinct 
wall, and gives off, at intervals between the septa, a number of 
radiating canals or tubuli (Fig. 1, #) which are seen to penetrate the 
wall of the siphuncle, and thus to communicate with the septal 
chambers, to which they may have conveyed blood-vessels, as 
suggested by Owen." 
The structure to which I desire now to direct particular attention 
is the perforation in the apex of the first segment of the siphuncle. 
There can be no doubt that this is strictly analogous to the per- 
foration in the apex of the siphuncle in Piloceras,’ and that through 
this opening the endosiphon passed from the initial chamber into the 
siphuncular cavity. 
The rare preservation of the apical extremity of any of the straight 
shelled Cephalopods of the genera Orthoceras, Endoceras, Actinoceras, 
or Piloceras, imparts a peculiar interest and value to the specimens 
which are the subject of the latter part of this note. 
It is to be hoped that by diligent search other specimens may 
yet be found which will throw more light upon these remarkable 
structures. 
1 Paleontology, 1860, p. 85. 
* See paper on this genus by the present writer in the GrotocicaL Macazine 
for December, 1887. 
