404 



THE DEVONIAN PERIOD 



A most extraordinary-looking creature is Pterichthys (Fig. 140), 

 in which the head and most of the body are encased in heavy 

 plates, the remainder in overlapping scale-like bones ; the eyes are 

 even closer together than in Cephalaspis. Dorsal and tail-fins are 



Fig. 140. — Pterichthys testudinarius. (From Dean, after Smith Woodward.) 



present and what appear to be pectoral fins. The pair of append- 

 ages referred to doubtless acted as fins, but they are not com- 

 parable to the paired fins of the true fishes, being merely jointed 

 extensions of the head-shield. These three genera, Cephalaspis, 

 Pteraspis, and Pterichthys , have been selected as types of the Os- 

 tracoderms, each one of which has several allies, differing from it 

 in one or other particular. 



Of the true Fishes there is great variety in the Devonian. The 



Fig. 141. — Cladoselache Fyleri, 1/5. (Dean.) 



Selachians are well represented, one of which is Cladoselache 

 (Fig. 141), a small shark, from two to six feet in length, and 'the 

 most primitive known member of the group. The Dipnoi, or 

 Lung Fishes, were important elements of the Devonian fish fauna, 



