ii4 HISTORICAL PALEONTOLOGY. 



internal skeleton, or if they have an external shell, it is not 

 chambered; their "arms" are furnished with powerful organs 

 of adhesion in the form of suckers ; and they possess only a 

 single pair of gills. For this last reason they are termed the 

 " Dibranchiate " Cephalopods (Gr. dis, twice ; branghia, gills). No 

 trace of the true Cuttle-fishes has yet been found in Lower 

 Silurian deposits; but the Tetrabranchiate group is represented 





Fig. 55. Fragment of Orthoceras ere- 

 briseptum, Cincinnati Group, North 

 America, of the natural size. The lower 

 figure is a section showing the air-cham- 

 bers, and the form and position of the 

 siphuncle. (After Billings.) 



Fig. 56. Restoration of Orthoceras, 

 the shell being supposed to be divided 

 vertically, and only its upper part being 

 shown, a. Arms ; /, Muscular tube 

 ("funnel") by which water is expelled 

 from the mantle-chamber - y c, Air-cham- 

 bers ; s, Siphuncle. 



by a great number of forms, sometimes of great size. The prin- 

 cipal Lower Silurian genus is the well-known and widely- 

 distributed Orthoceras (fig. 55). The shell in this genus agrees 

 with that of the existing Pearly Nautilus, in consisting of num- 



* This illustration is taken from a rough sketch made by the author 

 many years ago, but he is unable to say from what original source it was 

 copied. 



