NAUTILOIDEA. 



837 



Cambrian (Upper Tremadoc beds of Wales), and very numerous 

 forms of the genus have been recorded from the Ordovician rocks. 

 The genus attains its maximum in the Silurian, more than five 

 hundred species having been described by Barrande from rocks of 

 this age in Bohemia. In the Devonian and Carboniferous rocks a 

 great reduction in the number of species takes place ; a few Permian 



Fig. 747.— a, A broken specimen^ of 

 Orthoceras ornatnm, of the natural size, 

 from the Silurian rocks of Gotland ; b, 

 Transverse section of the shell, showing 

 the position of the siphuncle ; c, Portion 

 of the surface, enlarged. (After Foord.) 



Fig. 748. — a, Cast of the shell 

 of Orthoceras Hisingeri, from the 

 Silurian rocks of Gotland, of the 

 natural size ; b, Transverse section 

 of the shell, showing the siphuncle ; 

 c. Portion of the surface, enlarged. 

 (After Foord.) 



types are known ; and the last representatives of the genus are found 

 in the Alpine Trias. 



Family 2. Endoceratid^. — In this family the shell is straight 

 or slightly curved ; the siphuncle is marginal or sub-marginal, of 

 large size ; and the septal necks are invaginated so as to form a wide 

 tube within which the siphuncle is contained. 



