476 FISHES CHAP. 



bladder. The gill-filaments project freely beyond the outer edges 

 of the greatly reduced interbranchial septa. The external open- 

 ing of each nasal sac is usually divided into two distinct aper- 

 tures, and there is no oro-nasal groove leading from the sac to 

 the mouth. The brain has no proper cerebral hemispheres, but 

 retains an undivided prosencephalon with a non-nervous roof 

 A cloaca is not developed, the rectum opening externally by an 

 anus in front of, and distinct from, the separate or united urino- 

 genital apertures. The ova are small and numerous, and the 

 segmentation is either holoblastic and unequal, or meroblastic. 

 Besides a large number of fossil forms the group includes the 

 vast majority of living Fishes. 



The Teleostomi include four " Orders," the Ceossoptekygii, the 

 Chondrostei, the Holostei, and the Teleostei. Of these the 

 Crossopterygii occupy a remarkably central position. Eemotely 

 connected with the Elasmobranchs on the one hand, and more 

 intimately related to the Holostei and Teleostei on the other, 

 they also probably represent the ancestral stock from which 

 the Stegocephalan Amphibia and the Dipneusti have had their 

 origin. Of the three remaining groups, often collectively spoken 

 of as " Actinopterygii," the Chondrostei are the oldest and 

 most primitive. Like the Crossopterygii, they are not without 

 evidence of a remote kinship with the Elasmobranchs, but in a 

 broad general sense they also represent the initial stages in a 

 sequence of structural modifications, of which the Teleostei, the 

 dominant Fishes of the present day, are the final outcome. 



Order I. Crossopterygii. 



Pectoral fins obtusely lobate and probably uniserial, or acutely 

 lobate and probably biserial. Pelvic fins abdominal in position, 

 uniserial, non-lobate, or obtusely lobate. Scales rhombic or cycloid, 

 and, like the dermal cranial bones, they are generally invested by 

 a layer of enamel-like ganoin. Tail heterocercal, or apparently 

 diphycercal or gephyrocercal. Vertebral column acentrous, or with 

 ring-like centra, or even with complete bony amphicoelous centra. 

 Lower jaw with dentigerous splenials. As a rule, the opercular 

 series includes an operculum and a suboperculum. Branchio- 

 stegal rays absent, their place being taken by a remarkable 

 armature of jugular plates (Fig. 274). Secondary pectoral girdle 



