30 THE LESSON OF EVOLUTION 



All classes of the MoJlusca increased greatly, 

 especially the Cephalopoda, which are, after the 

 Brachiopods and Trilobites, the most numerous of 

 Silurian fossils. Now, too, we find, in the Lower 

 Ordovician, thick-shelled Gastropods, and in the 

 Silurian we have Chiton, an ancient form of soft- 

 bodied mollusc, specially modified for protection 

 among the waves of the shore. It has been sug- 

 gested on very good authority S. P. Woodward, 

 H. von Jhering, and Professor A. Hyatt that Ten- 

 taculites, and, perhaps, Hyolithes, represent the 

 primitive Cephalopoda. Anyhow, it is highly 

 probable that the first Cephalopods were pelagic in 

 habit, for we know no ground-animals from which 

 they could have been derived. These pelagic Cepha- 

 lopods are but little known, and possibly some of the 

 conodonts belonged to them. The ground 

 Cephalopods appear first as Nautiloidea, which 

 were very rare in the Upper Cambrian, increased 

 rapidly in numbers during the Ordovician, and 

 attained their maximum development in the 

 Silurian. 



Marine Arachnida are represented by the Euryp- 

 terida in the Ordovician, to which the Xiphosura 

 were joined in the Silurian. All the Xiphosura of 

 the Paleozoic era show a remarkable resemblance 

 to the Trilobites, as also does the young of the 

 modern Limulus, or King Crab. But as the latter 

 gets older the cephalic shield becomes united to that 

 of the thorax, and seven of the abdominal segments 

 also fuse together in an abdominal shield ; while the 

 primary division into three longitudinal lobes gets 



