182 SUB-CLASS GAXOIDEI. 



half-vei'tebrae, of which one half (variously described as the anterior and 

 posterior) carries the arches. Fulcra absent. Tail somewhat heterocercai, 

 more so in the young. 



The scales of Amia, which are thin, elastic and cycloidal, consist of a 

 superficial layer containing bone corpuscles, and a deeper layer containing 

 fibres. They thus closely resemble the scales of Teleostei, and are without 

 a superficial layer of ganoin (Klaatsch, op. cit., p. 179). 



The skull* is decidedly Teleostean in its structure. The chondrocraniiun 

 is well developed and contains a few widely separate cartilage bones. 

 As in Lepidosteus, it is without fenestrae in the roof. From the fact that 

 two neural arches are attached to the basioccipital bone, it would appear 

 that the centra of the two first vertebrae are fused to the skull. The 

 investing bones of the dorsal siu'face, though closely applied to the carti- 

 lage, are dermal structvu'es, and are for the most part not covered externally 

 by soft skin in the adult. The vomer is double and bears teeth. The 

 suspensorial apparatus is almost exactly similar to that typical of Teleos- 

 teans. The premaxillae, maxillae, palatines, and pterygoids also bear 

 teeth. There are four opercular bones, numerous branchiostegal rays, and 

 a median jugular plate. The shoulder girdle is cartilaginous and is over- 

 laid by the large clavicle. It carries one basal cartilage (metapterygium), 

 to one side of which the somactids, which carry the dermotrichia, are 

 attached. Spiracle, pseudobranch and hyoid gill are absent. There are 

 four double gills and a slit behind the fom-th arch. The conus arteriosus 

 has three transverse rows of valves with four or five valves in each row. 

 The base of the ventral aorta is slightly swollen into a kind of bulbus. The 

 air-bladder is cellular and lung-like and opens by a duct into the dorsal 

 wall of the pharynx. Stomach with a blind sac, pyloric caeca absent. 

 The intestine contains a spiral valve. For vu'inogenital organs, see p. 106. 

 A single living genus Amia L., with one species, in the freshwaters of the 

 United States ; flesh valueless as food ; species are known in the Lower 

 Tertiaries of Eiu'ope and N. America. Megalurus Ag., with fulcra, from 

 the Upper Jurassic may be placed here. 



Fam. 3. Oligopleuridae. Upper Jiu-assic to Upper Cretaceous. Oenos- 

 copus Costa, Spathiurus Davis. 



* Bridge, Cranial Osteology of Amia calva, Journ. Anaf. and Phys., 

 .., 1877, p. 605. 



