bo MANUAL OF THE MOLLUSCA. 



Orthoceras, Bre)Ti. 



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



Spi., cj'cloceras, McCoy. Gouioceras, Hall.* 



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



Shell, straight ; siphuncle central ; aperture sometimes contracted. 



Fossil, 125 typical sp. (D'Orb).t L. Silm-iau — Trias ; N. America, Aus- 

 tralia, and Eui'ope. 



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

 old rocks, and attained a lai'ger size than any other fossil shell. A fragment 

 of 0. giganteum, in the collection of JVIi-. Tate of Alnmck, is a yard long, and 



1 foot in diameter, its original length must have been 6 feet. Other species, 



2 feet in length, ai-e only 1 inch m diametei", at the aperture. 



Suh -genus I. Cameroceras, Conrad (= meha and thoracoceras, Fischer?). 

 Siphuncle lateral, sometimes ver}^ large {simple r). 

 Casts of these large siphimcles were called hyolites by Eichwald. 

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



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



2. Jctinoceras (Eronn), Stokes. Siphuncle very large, inflated between 

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

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



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

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

 America. 



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

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



* Theca and Tentaculites are provisionallj^ placed with the Pleropoda, they proba- 

 bly belong here. 



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

 Bohemia. 



I Fig. 47. Actinoceras Ricliardsojii, Sto\ie&. Lake Winipeg (diagram, reduced ^). 

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

 Geol. Trans.) 



