THE PROGRESS OF LIFE 65 



in which the triangular opening for the peduncle 

 remains open all through life, gave rise to the Ehyn- 

 chonellidce, which has a pair of deltidial plates in the 

 opening, and to the Strophomenidce, in which the 

 opening becomes, during growth, entirely closed by a 

 shelly plate, thus leaving the animal free. From the 

 Ehynchonellidce sprung, in the Silurian period, the 

 Terebratulidce, in which the deltidial plates remain 

 separate, and the Spiriferidce, in which they unite 

 during growth, and close the opening for the peduncle, 

 as in the Stropliomenidce. 



All classes of the Mollusca increased greatly, espe- 

 cially the Cephalopoda, which are, after the Brachio- 

 pods 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 suggested on very good authority S. P. 

 Woodward, H. von Jhering, and Professor A. Hyatt 

 that Tentaculites, and, perhaps, Hyolitlies, 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 



E 



