330 AMIOIDEI 



cranium persists to a considerable extent, yet most of the bones 

 typical of this group are well represented (Allis [10, 11], Franque 

 [138], Bridge [51], Shuffelt [409]). In the extinct genera the cheek 

 is usually covered by special plates, and the circumorbital bones are 

 more complete. There is a small eye-muscle canal in Amia and the 

 Semionotidae, and only two supratemporals (extra-scapulars). Two 

 free occipital neural arches overlie the foramen magnum, the 

 corresponding centra of which have been included in the basi- 

 occipital. The maxilla projects freely backwards and bears a 

 supra-maxilla. The opercular bones are well developed. There 

 is a single median gular, possibly homologous with the large paired 

 inferior gulars of the lower Osteichthyes (Fig. 312). The scales 

 are of the lepidosteoid type. 



Amia has no spiracular opening or gill, nor any opercular gill. 

 There is a considerable bulbus arteriosus (Fig. 69, p. 110), and paired 

 pulmonary arteries (p. 226). The viscera in Amia remain in an 

 unspecialised condition. The air-bladder is cellular, there are no 

 pyloric caeca. The urinogenital organs will be dealt with below 

 (p. 366). The cleavage of the egg is holoblastic, and the larva 

 has preoral adhesive organs. 



According to A. S. Woodward, the Amioidei include several 

 families which have diverged along two different main lines. The 

 oldest, the Semionotidae, and the possibly related Macrosemiidae, 

 would lead to the highly specialised Pycnodontidae with deep com- 

 pressed bodies and grinding teeth. From the other branch would 

 arise the predaceous sharp-toothed families Eugnathidae, Pachy- 

 cormidae, an.d Amiidae. That the last three families are closely 

 related there can be scarcely any doubt ; the general proportion of 

 the body, the fins, the vertebral column, the skull, etc., all support 

 this view. But the affinities of the Pycnodontidae are very doubtful. 

 It may also be doubted whether the Semionotidae are really more 

 closely related to the Amioidei than to the Lepidosteoidei ; certain 

 specialisations in the structure of the skull would seem to bring 

 them nearer to the latter. Lepidotus is possibly not an Amioid, 

 but a primitive Lepidosteoid. 



Family EUGNATHIDAE. Triassic, Jurassic, and Cretaceous fish of 

 elongate shape, with deeply forked caudal and short dorsal and anal. 

 The mouth is wide and the hyomandibular directed backwards. The 

 scales and cranial bones are ' ganoid,' there are small teeth on the palate,, 

 large pointed teeth on the jaws and the vomers (sometimes fused). 

 The fulcra are biserial, the scales thick and rhomboid, with a dorsal 

 articular peg (Eugnathus\ or thin and almost cycloidal (Gaturus, Eury- 

 cormus). The notochord may be persistent, without centra (Caturus) ; but 

 generally with vertebrae composed of separate post- and precentra (hypo- 

 and pleurocentra). The centra of Neorhombolepis and others are probably 

 formed by the fusion of these elements. 



