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 sae 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 CROSSOPTERYGII, the 
CHONDROSTEI, the HOLOSTEI, and the TELEOSTEI Of these the 
Crossopterygli occupy a remarkably central position. Remotely 
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 “ Actinopterygu,” the Chondrostei are the oldest and 
most primitive. Like the Crossopterygu, 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 subopereulum. Branchio- 
stegal rays absent, their place being taken by a remarkable 
armature of jugular plates (Fig. 274). Secondary pectoral girdle 
