318 



PALEOZOIC SYSTEM OF ROCKS. 



shells, with simple sutures and a central or subcentral siphon-tube 

 (siphuncle). The siphuncle of the family was large in proportion to 



a b 



Fig. 361 



Fig. 361.— Showing Structure of Orthoceratite. a, Ormoceras ; b, Actinoceras ; c, Huronia : d, Sec- 

 tion of Siphuncle of Huronia. 



Fig. 3G2.— Kestoration 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; c, air-chambers; s, siphuncle (after Nicholson). 



the shell, and had often a beaded structure (Fig. 361, «, #, c, d). The 

 genera are founded largely on the form of this part. 



They existed in great numbers, and attained very great size. Speci- 

 mens have been found fifteen feet long, and eight to ten inches in diam- 

 eter. They were, without doubt, the most powerful animals of that 

 time, the tyrants and scavengers of these early seas. We give, in Fig. 

 357, a restoration of the creature. They are entirely characteristic of 

 the Palaeozoic ; commencing in the Primordial, extending through into 

 the Carboniferous, and passing out there. They attained their maxi- 

 mum of devolopment in size and number in the Silurian. 



Although straight-chambered shells (Orthoceratites) are most abun- 

 dant and characteristic, and also the earliest, coiled shells of the same 

 tribe (Nautiloids) are also found, and some of them of considerable 

 size, but not until the upper Silurian. Some of these are close-coiled 

 shells, true Nautilus family ; others open-coiled, and more nearly allied 



