494 



MANUAL OF ZOOLOGY. 



merce, whilst the roe is largely employed as a delicacy under the name of 

 caviare. In the Paddle-fishes (Spatiilaria} the skin is not provided with 

 an exoskeleton. Both Spatularia and Scaphirhynchiis are found in the 

 rivers of North America ; but two species of the latter have recently been 

 discovered in Asia. 



Only a few fossil forms belonging to the Sturionida are at present known ; 

 and by far the greater number of extinct Placoganoids belong to the family 



Ostracostei, established by Owen, and characterised by the fact that the 

 head, and generally the anterior part of the trunk as well, was encased in 

 a strong armour composed of numerous large ganoid plates, immovably 

 joined to one another. The posterior extremity of the body was more or 

 less completely unprotected, and, whilst the notochord was persistent, the 

 peripheral elements of the vertebrae namely, the neural and haemal spines 



may be ossified. 



Fig 267. Cephalaspis Lyellii, 



The Ostracostei^ or " Placoderms," are entirely confined to the De- 

 vonian and Upper Silurian rocks, and the most important genera com- 

 prised in the group are Cephalaspis (fig. 267), Pteraspis, Fterichthys, and 

 Coccosteus. 



CHAPTER LV. 

 ELASMOBRANCHII AND DIPNOI. 



ORDER V. ELASMOBRANCHII ( = Selachia, Miiller; Placoidei, 

 Agassiz ; Holocephali and Plagiostomi, Owen). This order in- 

 cludes the Sharks, Rays, and Chimserae, and corresponds with 

 the greater and most typical portion of the Chondropterygidce or 

 Cartilaginous fishes of Cuvier. The order is distinguished by 

 the following characters : The skull and lower jaw are well 

 developed, but there are no cranial bones, and the skull consists 

 of a single cartilaginous box, without any indication of sutures. 

 The vertebral column is sometimes composed of distinct vertebra, 



