VERTEBRATA 403 



noids, of which Goniatites (IV, 10; V, 11) is the common form, 

 the sutures are much less complex than in the Mesozoic shells. 

 Another member of the group which is far more abundant in 

 Europe than in America is Clymenia, the only Ammonoid in 

 which the siphuncle is on the inner side of the spiral. Bactrites 

 has a straight shell, like that of Orthoceras, but with the com- 

 plex sutures which show it to be an Ammonoid. 



Vertebrata. — One of the most characteristic features of Devo- 

 nian life is the great development of the aquatic Vertebrates, 

 which is so striking that the period is often called the " Age of 

 Fishes." So numerous and so finely preserved are these fossils 

 that a satisfactory account may be given of the structure and sys- 

 tematic position of many of the genera. This great assemblage 

 of fishes and fish-like forms, it should be remembered, is not 

 something entirely new in the earth's history, but the wonderful 

 expansion of types which during the Ordovician and Silurian had 

 remained very much in the background. 



Of the Devonian Vertebrates none are more peculiar and char- 

 acteristic than the Ostracodenns, which, though generally called 

 fishes, really belong to a type much below the true fishes and 

 more nearly allied to the Lampreys, being devoid of jaws and of 

 paired fins. The head and more or less of the body are sheathed 

 in heavy plates of bone, and the remainder of the body and the tail 

 are covered with scales. No trace of the internal skeleton is pre- 

 served, and it evidently was not ossified. The genus Cephalaspis 

 of this group is curiously like a Trilobite in appearance, though, 

 of course, the resemblance is entirely superficial. The head- 

 shield is formed of a single* great plate of bone, shaped like a sad- 

 dler's knife, with rounded front edge and with the hinder angles 

 drawn out into spines • the eyes are on the top of the head and 

 very close together. The body is covered with large, angular 

 plates of bone, arranged in rows ; a small median dorsal fin and 

 a larger triangular tail-fin make up the locomotor apparatus. 



Pteraspis has a bony plate over the snout, a large shield on the 

 back and another on the belly, with rhomboidal scales covering 

 the rest of the body, 



