4 I4 THE ANIMAL KINGDOM. 



it 'does in Acipenser. The praemaxillae are absent in the Sturgeons : 

 the maxillae are represented by a chain of bones in Lepidosteus^ and both 

 praemaxillae and maxillae are absent in Dipnoi. The mandible is invested 

 by a dentary bone alone in Sturgeons : by dentary, splenial, and angular 

 in Dipnoi^ and Polyptems, to which Lepidosteus and Amia, add a supra- 

 angular and the former a coronoid. As to opercular bones, Acipenser has 

 a large operculum : the Dipnoi have opercular and interopercular bones ; 

 the inter-operculum is absent in Polypterus and the prae-opercular fused 

 with the squamosal : the prae-operculum is very small in Lepidosteus^ the 

 interoperculum of great size. Amia resembles a Teleostean. Amia has 

 a single, Polypterus a double jugal bone between the mandibular rami. 

 These bones are also found in some extinct Ganoidei, and must not be 

 confounded with the branchiostegal rays of Amia and Teleostei, which carry 

 on the series of opercular bones ventrally. 



The hyoidean and branchial arches are unossified 'in Elasmobranchii, 

 Holocephali, Ceratodus, and Protoptems. The hyoid is an unsegmented rod 

 in the Dog-fish (Scy Ilium) and some other Sharks. It and the branchial 

 arches are as a rule segmented and ossified typically (p. 93). The last 

 arch present is usually rudimentary. There are seven branchial arches 

 in HeptanchuS) six in Hexanchus and Protopterus, five in other Fish. The 

 fifth pair fuse into a median bone in Pharyngognathi (Teleostei). Gill- 

 rakers or pointed cartilaginous processes crossing the branchial slits are 

 borne upon the branchial arches in many Elasmobranchii, some Teleostei 

 and the Dipnoi. Branchial rays radiate outwards from the branchial arches 

 into the septa between the gill-pouches in Elasmobranchii : and in Sharks 

 curved extra-branchial cartilages are attached to the bases of more or 

 fewer of the branchial arches and He close beneath the outer edges of the 

 septa just mentioned. 



The backbone is formed by the notochord with its sheath in a few 

 Elasmobranchii (e. g. Echinorhinus\ in Holocephali, chondrostean Ganoidei, 

 Dipnoi. Bony rings, more numerous than the arches, are formed in the 

 sheath in Holocephali. In other Fish there are ossified amphicoelous 

 vertebrae, between which the notochord persists. Lepidosteus alone has 

 opisthocoelous vertebrae, and in development an inter-vertebral thickening 

 of the cartilage which divides, forming, as in Sauropsida and Amphibia, the 

 opposing faces of adjoining vertebrae. The vertebral centra originate in 

 Elasmobranchii by the growth of the bases of both neural and haemal 

 arches round the notochordal sheath, and the fusion of the growths with 

 the vertebral thickenings of the sheath. Ossification in the centra thus 

 formed takes place concentrically (Cyclospondylt) or radially (Asterospondyli}. 

 The centra in bony Ganoidei and Teleostei are formed chiefly by parosteal 

 ossification, imbedding the bases of the arches neural and haemal (p. 100). 

 The centra are united at their edges by ligament. The neural ridges form 



