88 



MANUAL OF THE MOLLUSCA. 



Orthoceras, BrejTi. 



Etym., ortJios, straight, aud ceras, a horn. 



Si/n., cycloceras, McCoy. Gouioceras, Hall.* 



Ex. O. giganteuin (diagram of a longitudinal section), pi. II, fig. 14. 



Shell, straight; siphnncle central; apertiu-e sometimes contracted. 



Fossil, 125 tj^iical sp. (D'Orb).t L. Silmian — Trias ; N.America, Aus- 

 txalia, and Em-ope. 



The orthocerata are the most abundant and \N-ide 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 ^Ir. 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?). 

 Siphunde lateral, sometimes %ery lai'ge {simple r). 

 Casts of these large siphuncles were called hyolites by Eichwald. 

 27 sp. L. Silurian — Trias ? N. America and Em-ope. 



Fig. 47. Actinoceras.X Fig. 48. Ormoceras. 



2. Actlnoceras (Erouu;, Stokes. Siphnncle very large, inflated between 

 the chambers, and connected \A'ith a slender central tube by radiating plates. 

 6 sp. L. Silm-ian — Carb, N. America, Baltic, and Brit. 



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

 the septa appeal- as if united to the centre of each). 3 sp. L. Silm-ian, N. 

 America. 



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

 Siphnncle very large, central, the upper part of each joint inflated, coimected 



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

 bly belong here. 



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

 Bohemia. 



t Fig. 47. Actlnoceras Richardsoni, Stokes. Lake Winipeg (diagram, reduced §). 

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

 Geol. Trans.) 



