MANUAL OF THE MOLLUSCA. 



OKTHOCERAS, Breyn. 



Etym., orthos, straight, and ceras, a horn. 



Syn., cycloceras, McCoy. Gonioceras, Hall.* 



Ex. 0. giganteiun (diagram of a longitudinal section), pi. II, fig. 14. 



Shell, straight ; siphuncle central ; aperture sometimes contracted. 



Fossil, 125 typical sp. (D'Orb).f L. Silurian Trias; N.America, Aus- 

 tralia, and Europe. 



The orthocerata are the most abundant and wide spread shells of the 

 old rocks, and attained a larger size than any other fossil shell. A fragment 

 of 0, giganteum, 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 I. Cameroceras, Conrad (= melia and thoracoceras, Fischer?). 

 Siphuncle lateral, sometimes very large (simple ?} . 

 Casts of these large siphuncles were called hyalites by Eichwald. 

 27 sp. L. Silurian Trias ? N. America and Europe. 



xl 



Fig. 47. Actinoceras.l Fig. 48. Ormoceras. 



2. Actinoceras (Bromij, Stokes. Siphuncle very large, inflated between 

 the chambers, and connected with a slender central tube by radiating plates. 

 6 sp. L. Silurian Garb, N. America, Baltic, and Brit. 



3. Ormoceras, Stokes. Siphuncular beads constricted in the middle (making 

 the septa appear as if united to the centre of each). 3 sp. L. Silurian, N. 

 America. 



.4. Huronia, Stokes. Shell extremely thin, membraneous or horny ? 

 Siphuncle very large, central, the upper part of each joint inflated, connected 



* Theca and Tentaculites are provisionally placed with the Pteropoda, they proba- 

 bly belong here. 



t M Barrande has discovered 100 new species in the Upper Silurian rocks of 

 Bohemia. 



I Fig. 47. Actinoceras Richardsoni, Stokes. Lake Winipeg (diagram, reduced |). 

 Fig. 48. Ormoceras, Bayfieldi, Stokes. Drummond Island, (from Mr. Stokes' paper, 

 Geol. Trans.) 



