Ichthyopsida 425 



whose genera] arrangement resembles that of the bones of the skull in 

 modern fishes. A large hyomandibular bone, extending from otic 

 region of cranium to the quadrate bone, assists in supporting the jaws. 

 Degree of vertebral ossification variable; centra usually incomplete 

 or lacking; presumably a persistent notochord. Caudal fin heterocercal 

 or diphycercal (Fig. 327). 



Scales are strongly developed, each consisting of a thick plate of 

 bone overlaid by a thick layer of calcareous substance, cosmin, 

 resembling dentine in structure. The cosmin is covered externally by 

 a very thin layer of a harder enamel-like substance, ganoin. Scales of 

 this type are called "cosmoid" (Fig. 328). 



External apertures of each lateral series of gill-clefts are covered 

 by an operculum (Fig. 329C), a backwardly projecting fold of skin 

 stiffened by thin plates of dermal bone and free at its posterior edge 

 to allow egress of water from the gill-chambers. Internal nares 

 (choanae) piercing anterior roof of mouth indicate possession of lungs 

 as accessory respiratory organs. 



The especially distinctive characteristics of the group are the 

 lobe-fins, cosmoid scales, choanae, and incomplete vertebral 

 ossification. 



The crossopterygians (Fig. 329C), primitive fresh-water fishes, were 

 at their height in the Devonian, but steadily diminished during the 

 Mesozoic, and only one genus (Latimeria) , so far as is known, now 

 exists. 



SUBCLASS IV. ACTINOPTERYGII 



Endoskeleton partly or completely bony. Embryonic noto- 

 chord usually replaced by a bony vertebral column. Vertebrae usualh 

 amphicoelous. Scales, sometimes lacking, are calcareous and of several 

 structurally different types but rarely placoid and never cosmoid. 

 Gill-clefts (exclusive of spiracles) never in excess of five pairs; some- 

 times fewer. External branchial apertures of each side covered by 

 an operculum. No cloaca. 



Spiracles, the spiral valve, and the heterocercal tail-fin do not 

 appear in the great majority of adult modern actinopterygians, but in 

 a small minority of them and in many extinct members of the group 

 these three elasmobranch features occur, together with other character- 

 istics which are reminiscent of sharks. 



An especially prominent feature of most actinopterygians is an 

 organ whose equivalent is not to be found among sharks. This is the 

 air-bladder or " swim-bladder." It is a thin-walled sac, single or paired, 

 which develops as a hollow outgrowth from the wall of the pharynx or 

 from a region of the digestive tube more or less posterior to the pharynx 

 (Fig. 331). In the adult the sac may or may not retain its original open 



