42 THE LESSON OF EVOLUTION 



and 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 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 Elasmobranchii are true fishes, represented in 

 our seas by the sharks and rays. The primitive 

 Elasmobranchs, called Ichthyotomi, are limited to 

 the Deutozoic era. In them the notochord was 

 formed by a cylindrical rod of equal thickness, 

 sheathed with cartilage, which was very slightly 

 calcified and was not constricted into vertebrae. 

 Also the pectoral fins had a segmented axis, like 

 those of the Dipnoi. The early Selachii, from which 

 all our sharks and rays are descended, have the 

 sheath of the notochord more calcified and con- 

 stricted in the centre of each vertebra. The pectoral 

 fins have no segmented axis., and the skin is covered 

 with thorny scales. Of these, the sharks and dog- 

 fish, in which ossification of the vertebral column 



