332 ORDERS OF FISHES. 



whilst it is extended into a rostrum in front. The posterior 

 spine is very small, and is attached to the disc as a separate 

 piece. The orbits are situated laterally. 

 The minute structure of the shield is very 

 complex, consisting of three layers. The 

 innermost layer is laminated, and is tra- 

 versed by vascular canals. The middle 

 layer is made up of polygonal cavities ; 

 and the outer layer is structureless or 

 fibrous, and is finely striated or grooved. 

 The body was covered with scales ; but 

 nothing is known of the nature of the fins. 

 The genera Cyathaspis and Scaphaspis 

 have been founded upon forms which have 



e usuall 7 been P laced under Pteraspis, and 



of Ludiow. (After Mur- which differ in more or less essential 

 points from the typical species of this 

 genus. The genus Pteraspis, so far as yet known, comprises 

 the most ancient of the fishes, commencing as it does in the 

 earlier portion of the Ludiow formation (Upper Silurian). 

 Other species are known in the Old Red Sandstone ; but the 

 genus appears to have entirely disappeared before the close of 

 the Devonian period. 



In the genus Coccosteus the head was protected by a great 

 shield, the plates of which are covered with small hemispheri- 

 cal tubercles. There is also a ventral or " sternal " shield, 

 which, according to Huxley, seems to have had no direct con- 

 nection with the cephalic buckler. The mouth was furnished 

 with a distinct lower jaw or "mandible," composed of two 

 rami, carrying small teeth. The notochord was persistent, but 

 the neural and haemal spines of the vertebrae, and the rays of 

 the dorsal and anal fins, are well ossified. A heterocercal tail- 

 fin was doubtless present as well. The genus Coccosteus is 

 essentially Devonian ; but a species has been discovered by 

 M. Barrande in the Upper Silurian of Bohemia. 



In the genus Pterichthys (fig. 294) are some very remarkable 

 fishes, first discovered in the Old Red Sandstone by the late 

 Hugh Miller, and nearly related in most respects to Coccosteus. 

 The whole of the head and the anterior part of the trunk were 

 defended by a buckler of large ganoid plates suturally united, 

 those covering the trunk forming a back-plate and a breast- 

 plate articulated together at the sides. The rest of the body 

 was covered with small ganoid scales. A small dorsal fin, a 

 pair of ventrals, a pair of pectorals, and a heterocercal tail-fin 

 were present The form of the pectoral fins is the peculiar 



