402 THE DEVONIAN PERIOD 



istic of the Devonian Trilobites is the extraordinary development 

 of spines which many display on the head- and tail-shields. 



The other Crustacea make notable progress in this period. 

 The first of the Isopoda and of the long-tailed Decapoda (lobster- 

 like forms) make their appearance in the Devonian. The Euryp- 

 terids now attain their culmination in size, being actually gigantic 

 for Crustacea, and some of them are as much as six feet long. 

 The genera (Eurypterus, Stylonurus, and Pterygotus) are the same 

 as in the Silurian. Insects, though still rare as fossils, are very 

 much commoner than in the Silurian ; they represent both Or- 

 thopters and Neuropters, which are among the primitive groups. 



Brachiopoda. — As in the Silurian, Brachiopods continue to be 

 the most abundant fossils, both in species and individuals, in 

 the Devonian, from which more than iooo species have been 

 described. Many Silurian genera have died out, and others, like 

 Orthis and Strophomena t have become much less common ; and 

 of others again, like Chonetes and Productus, the species are more 

 numerous. The most characteristic shells are those belonging to 

 the genera Spirifcra, especially the very broad "winged" species, 

 (IV, 3), Rhynchonella (IV, 5), Athyris (IV, 4), and those belong- 

 ing to the still existing family. Terebratulidce, of which Rensellaria 

 and Stringocephalus are Devonian genera. 



Mollusca. — Bivalves and Gastropods are much as in the Silu- 

 rian : examples of the former are Pterinea (IV, 6) and Conocar- 

 dium (IV, 7), while large species of the Gastropod Euomphalus 

 (IV, 8) are characteristic. The Cephalopods have been revo- 

 lutionized ; the wealth of Nautiloid shells which we found in the 

 Silurian has been much diminished, though Orthoceras, Phrag- 

 moceras, Gomphoceras (IV, 9), and Cyrtoceras still persist, but 

 with fewer species than before, while many other genera have 

 disappeared. More significant is the first appearance of the 

 Ammonoid division of the Tetrabranchiate Cephalopods, a group 

 of shells which was destined to attain extraordinary development 

 in the Mesozoic era. The Ammonoids are distinguished by the 

 complexity of the " sutures," or lines made by the junction of the 

 septa with the outer wall of the shell. In the Devonian Ammo- 



