THE PROGRESS OF LIFE 75 



DEUTOZOIC LIFE 



The two great features of the life of the Deutozoic 

 era are the abundance of land vegetation and the first 

 appearance of air-breathing vertebrates ; but, before 

 saying more about them, we will first take a glance at 

 the fishes. 



The armour-plated Ostracodermi of the Silurian 

 became numerous in the Devonian. These curious 

 animals are of very uncertain affinity, and by some 

 naturalists are excluded from the class of fishes alto- 

 gether, and placed alongside the Lampreys ; because, 

 like them, they had no lower jaws ; while Mr Traquair 

 considers them to be true fishes, descended from some 

 primitive Elasmobranch. Their internal skeleton was 

 entirely cartilaginous, but they had a strong armour 

 of calcareous plates over the anterior part of the body, 

 and scales on the posterior part. In Cephalaspis and 

 Pteraspis, which commenced in the Silurian, the 

 ventral shield is simple, and the dorsal shield is 

 either simple or is formed of a few plates, which 

 unite firmly together in the adult. But in the later 

 forms (Pterichthys) both ventral and dorsal shields 

 were formed by several bony pieces covered with 

 enamel, and they had pectoral appendages which were 

 also encased in armour. All of them became extinct 

 at the close of the Devonian period. Many specimens 

 of what is thought to be a Lamprey (Palaeospondylus) 

 have been found in Scotland ; but this animal, although 

 not more than three inches in length, had well de- 

 veloped vertebrae, and was, therefore, more specialised 

 than the lampreys of the present day. 



The Masmobranchii are true fishes, represented in 



