20X ORTHOCERAS, CAMEROCERAS. ETC. 



droid, small, marginal, the siphonal investment more or less 

 solid mid persistent. 



Fossil only, about KJOO species (?) known. 



Family I. XAUT1LID.K. 



Genu.s ORTHOCERAS, Breyn. 



Sliell straight; aperture sometimes contracted. 



Fossil, 240 species. L. Silurian to Liassie ; N. America, Aus- 

 tralia, Europe. 



Probably the animal was not able to withdraw itself completely 

 into its shell, as in the Nautilus. That the shell was external is 

 indicated by the colored bands preserved on 0. anguliferus. 



These shells attained sometimes i>Te:it si/e ; a specimen in the 

 collection of Mr. Tate, of Almvick. England, must have been 

 six feet long when perfect. 0. Titan is estimated to have 

 weighed "some tons."* The aperture is sometimes so con- 

 tracted that species two feet in length have a diameter of only 

 one inch at the mouth. 



O. PLANICANALIOULAT1 >i. Sandb. PI. 9f>. fig. 470. Pevonian. 



Nassau. 

 O. SUB ANNUL ARE. Barr. PI. 9C>. fig. 471. Silurian. Bohemia. 



The following subgenera or groups are generally adopted : 

 Cameroceras, Conrad. 



Siphuncle lateral, sometimes very laroe (simple?). 



Twenty-seven species. T^. Silurian toTriassic? N.America, 

 Europe. 



C. VERMICULARIS, d'Arch. PL %. fig. 472. 

 C. VAGINATUS. Schloth. PI. 9fi, fig. 473. 



Actinoceras, (Brown). Sfokos. 



Siphuncle verv large, inflate*! between the chambers and con- 

 nected with a slender central tube bv radiating plates. 



Six species. L. Silurian to ( 1 arb. N. America, Europe. 

 \. KICRARDSOM. Stokes. PI. 9C>. fig. 474. L. Winnepeg. 



A. BlGSBYT. PI. '.('). fin-. 475. 



Newberry, Palaeont. Ohio, i, 263. 



