162 SUB-CLASS GANOIDEI. 



The structure of the skull is very varied, but the cartila- 

 ginous cranium always contains cartilage bones and is invested 

 by membrane bones. The dorsal membrane bones are dermal 

 structures and are frequently in the adult exposed on the surface 

 in consequence of the superjacent epidermis and dermis having 

 been rubbed off (see p. 161). The suspensorium is always 

 hyostylic. 



The shoulder girdle and pectoral fin skeleton presents great 

 variation, but on the whole inclines to the Teleostean type of 

 structure. In the Chondrostei and in Amia the pectoral girdle 

 is cartilaginous. In the other orders it is ossified very much as in 

 Teleosts. 



In the Chondrostei and Crossopterygii there are three separate 

 membrane bones in relation with the shoulder -girdle, the supra- 

 clavicle, clavicle and infra-clavicle,* while in the other orders the 

 infra-clavicle is absent. In the pectoral fin the skeleton of 

 Polypterus is on the Elasmobranch type (rhipidostichous), in 

 extinct Crossopterygians on the Dipnoan type (rachiostichous 

 and mesorachic). In other orders the arrangement on the 

 whole is Teleostean. 



The other anatomical characters of the Ganoids are (1) the 

 possession of a conus arteriosus with more than one row of 

 valves ; (2) the very general presence of a gill on the hyoid 

 arch supplied by a branch of the ventral aorta, or of a spiracle 

 (see below) ; (3) the presence of an air-bladder with pneumatic 

 duct ; (4) the union of the urinary and Mullerian ducts and their 

 opening by a median urogenital pore behind the anus ; (5) the 

 testis appears to be connected except in Polypterus with the 

 kidney by a testicular network ; (6) a spiral valve is present in 

 the intestine (small in Lepidosteus and Amia) ; (7) the optic 

 nerves form a chiasma ; (8) the openings of the nasal pits are 

 double as in Teleosteans ; (9) the processus f alcif ormis and choroid 

 glands are absent in Polypterus, Lepidosteus, and possibly in 

 other Ganoids ; (10) the scales are bony plates embedded in the 

 dermis, and frequently covered by a layer of peculiar substance 

 called ganoin. Ganoin is probably vitro-dentin : it used to be 



* These bones are now often called supracleithrum, clei thrum and 

 clavicle respectively, on the view that the last-named alone is homo- 

 logous with the clavicle in the Amphibia and Amniota, . the cleithral 

 elements not being represented in these groups. 



