THE AGE OF INVERTEBRATES OF THE SEA, 73 



they have been studied. In this formation alone about 450 

 species are known, and in the Silurian these increase to 1,200; 

 and here the group culminates. It returns in the Devonian to 

 about the same number with the Lower Silurian, diminishes 

 in the Carboniferous to 350, and in the Mesozoic, where 

 the Nautiloid forms are replaced by others of the type 



"i 1 



Kk;. (i-j.—Gomphoccras. 



Vv;. fiCy — Orthocerns. I-owe» Silurian. 

 'I'he dotifd lin- shows the position 

 of the siphuncle. 



Fi<;. 08. — Liluitcs. 



of the Ammonites, becomes largely reduced. In the Tertiary 

 there are but nineteen species, and, as already stated, in 

 the modern world three. These statements do not, however, re- 

 present the whole truth. In the Paheozoir, in addition to the 

 genus Nautilus, we have a great number of other genera, some 

 with perfectly straight siielLs, like Ortlioceras (Fig. 66), others 



