ACIIifOCEKATIDJi:. 
169 
Sp. Char. Shell straight. Section circular, or subcircular. Rate 
of increase somewhat rapid, about 1 in 7. Septa numerous, about 
3 lines distant where the diameter of the shell is 19 lines, 
increasing to 4 Hues where the diameter measures 26 lines. They 
have a very short recurved neck, and are united with the siphuncle 
up to a point just below its greatest expansion, where they 
leave it and sweep upwards till they reach the shell-wall. The 
sij)huncle is very large and greatly expanded between the sox^ta ; the 
ratio of the height of a segment to its breadth being nearly as 
4 : 13. The endosiphon is weU developed, and the tubuli connecting 
it with the siphuncular walls are xweserved in many specimens. In 
some examples from Thessalon Island, Lake Huron, the matrix 
filling the septal chambers has been dissolved away, exposing the 
endosiphon and tubuli completely to view, though their form is 
somewhat obscured by crystals of pearl-spar in crusting them. In 
section the endosiphon presents a very irregular outline, which 
suggests its originally membranous nature. Hall describes the 
outer shell as thin, and “ connected with an interior one which 
often closely joins it by infiltration of mineral matter, when it 
presents a thickened appearance.” The surface is “marked by 
longitudinal undulated fine thread-like lines.” 
liemarhs. This species has a curious history. It was first named 
by Bronn in 1837, as the type of his genus Actinoceras, having 
been previously (1824) well figured by Bigsby, who also gives a very 
lucid description of it, but without bestowing a name. Bigsby ’s 
description and figures supx)lied in fact the material for the 
establishment of Bronn’s genus. A. Bigshyi was next briefly 
characterized by Stokes, who distinguished it from A. Lyonii because 
of its having a larger tube [endosiphon] within the siphuncle. 
The ax)parent superiority in size is really due to the incrustation of 
crystalline matter already mentioned. The interior of the type 
specimen of the latter species is filled with calc-spar, and this 
accounts for the absence of any of the radiating tubuli (“ verticilla- 
tions ” of Stokes) which are always seen in well-preserved specimens. 
Their absence is remarked upon by Stokes as further evidence of the 
distinctness of his species. 
Under the provisional name of Ormoceras tenuijilum^ Hall de- 
scribed a form {loc. cit.) about whose identity with the present 
species there can be no doubt. That author, however, includes in 
the synonymy of his species Ormoceras Backii^ Stokes, remarking 
that “ the specimens of Mr. Stokes are from a locality in which the 
rock belongs to the lower stages of our system, and probably to a 
position precisely corresponding to the thin mass containing these 
