FISHES OF THE PHILIPPINE ISLANDS AND ADJACENT SEAS 511 



Series GANOIDEI 



Skeleton usually cartilaginous, sometimes very primitive. Air 

 bladder highly developed, usually cellular and functional as a lung. 

 but connects with upper side of gullet, not with ventral side as in 

 dipnoans. Optic nerves crossing before reaching eyes, thus form 

 more or less perfect chiasma. Arterial bulb of heart with many 

 valves. Intestine with spiral valve. Usually an armature of bony 

 plates present, diamond-shaped and with an enamel like that formed 

 on teeth. Tail usually distinctly heterocercal, if only obviously so. 

 Pectoral fin with numerous basal bones or actinosts. 



The ganoid fishes form a purely provisional assemblage, showing 

 many archaic features and therefore are grouped together nearest 

 the crossopterygians or fringe fin stock of fishes. The great range 

 of variation in structure and form likely indicate the remnants of a 

 vast host of primitive fishes derived from some of the fringe fins. In 

 Mesozoic seas ganoids were hardly less varied and perhaps scarcely 

 less numerous than the teleosts living in the seas today. In number 

 and variety of forms they greatly exceed the fringe fins. Of the six 

 orders usually admitted, four are still represented by a few living 

 forms. 



Order Chondrostei 



Teeth small or absent. Subopercle and preopercle absent. Bran- 

 chiostegals few or absent. Cartilaginous vertebrae imperfectly de- 

 veloped. Notochord persistent. Axinosts and baseosts of median 

 fins arranged in simple regular series, rays more numerous than sup- 

 porting elements. Pelvic fins with well-developed baseosts. Shoul- 

 der girdle with pair of infraclavicular plates. Optic nerves forming 

 chiasma. Intestine with spiral valve. Body naked or with longi- 

 tudinal rows of bony plates. Rhombic plates on tail. One dorsal 

 and one anal, distinct from caudal. Caudal usually heterocercal. 

 Pair of pectorals and pair of ventrals. 



A large group comprising nearly half of the extinct ganoid fishes 

 besides the few living sturgeons, now thought degenerate modern 

 forms. Families, three. 



Family ACIPENSERIDAE 



Body elongate, partly cylindrical. Head large, robust. Snout de- 

 pressed, extended, conic or partly spatulate, with sensory areolae 

 on lower surface. Eyes small. Mouth small, inferior, protractile. 

 Lips thick, produced into marginal lobes. No teeth, except micro- 

 scopic ones in larval stages. Maxillary distinct from premaxillary. 

 Four barbels in transverse row on snout below before mouth. Nos- 



