PALEOZOIC ROCKS AND ERA. 



281 



teropods, like conchs, whelks, etc., are also found; but, 

 in order to avoid confusing the mind with too many de- 



FIG. 190. Silurian brachiopods : Spirifer Cumberland!* a, ventral valve ; 6, suture. 



tails, we shall pass over these and confine ourselves only 

 to the most striking and characteristic forms. 



Ceplialopods ; Ortlioceratite. The great class of 

 Cephalopods, including now the squids, cuttle-fishes, and 

 nautilus, were represented, in Silurian times, by -a very 

 remarkable family called Orthoceratite (straight-horn). 

 The appropriateness of the name is recognized by the 

 figures on page 282 (Figs. 192, 193). 



Cephalopods now are, some of them, naked (squid and 

 cuttle-fish), and some shelled (nautilus). When they have 

 a shell, the shell is cham- 

 bered. The animal lives 

 in the outer part, and all the 

 chambers are empty, full 

 of air only, and connected 

 with the animal by a mem- 

 branous tube called the si- 

 phon-tube or siphuncle (Fig. 

 191). Now, at the present 

 time, nearly all Cephalopods 

 are naked. Only one genus 

 of the shelled kind remains, 

 viz., the Nautilus. In Silu- 

 rian times, and indeed long after, there were no naked 

 ones. Only the shelled kinds existed. The naked 



FIG. 191. Pearly nautilus (Nautilus 

 pompilius) : , mantle ; 6, its dor- 

 sal fold ; c, hood ; o, eye ; t, ten- 

 tacles ; /, funnel. 



