CEPHALOPODA. 



191 



The ortJwcerata are tlie most abundant and wide-spread Bhells 

 of the old rocks, and attained a larger size than any other fossil 

 shell. A fragment of an orthoceras, in the collection of Mr. 

 Tate of Alnwick, is a yard long, and 1 foot in diameter, its 

 original length must have been 6 feet. Other species, 2 feet in 

 length, are only 1 inch in diameter at the aperture. 



Sub-genus. 1. Gameroceras, Conrad (= meliaand thoracoceras, 

 Fischer ?). 



Siphunde lateral, sometimes very large {simple ?). 



Casts of these large siphuncles were called hyolites by Eichwald. 



27 species. Lower Silurian — Trias ? North America and 

 Europe. 



Fig. bi. Actinoceras.* 



Fig. 55. Ormoceras. 



2. Actinoceras (Bronn), Stokes. Siphuncle very large, in- 

 flated between the chambers, and connected with a slender 

 centi'al tube by radiating plates. 6 species. Lower Silurian — 

 Carb. ; North America, B*ltic, and Brit. 



3. Ormoceras, Stokes. Siphuncular beads constricted in the 

 middle (making the septa appear as if united to the centre of 

 each). 3 species. Lower Silurian — Devon; North America. 

 This sub-genus very much resembles, if it is not identical with, 

 the last mentioned. 



4. Huronia, Stokes. Shell extremely thin, membranous or 

 homy ? Siphuncle very large, central, the upper part of each 

 joint inflated, connected with a small central tube by radiating 

 plates. 3 species. Lower Silurian. Drummond Island, Lake 

 Huron. 



Numerous examples of this curious fossil were collected by Dr. 

 Bigsby (in 1822), and by the officers of the regiments formerly 



* Fig. 54. Actinoceras Richardsoni, Stokes. Lake Winipeg. (Diagram reduced ^.) 

 Fig. 55. Ormoceras Bayfieldi, Stokes. Drummond Island. (From Mr. Stokes' paper, 

 Geol. Trans.) 



