508 CHORD ATA. 



leus having opisthoccele vertebra.'. There are no characters in all ganoids 

 which occur only in the group. The ganoid scales are often absent, since 

 the sturgeon has bony plates free from ganoin, while the paddle bill 

 (Polyodon) has almost no dermal skeleton, and A mi a has cycloid scales. 

 Most recent and fossil forms possess fulcra, bony plates with forked ends 

 lying shingle-like in front of the fins (rig. io,5), but these are frequently 

 absent, e.g., in Amia and Polyptems (fig. 10, A and C). The group is 

 largely American. The few recent ganoids fall into three distinct groups. 



Order I. Crossopterygii. 



Largely extinct, two genera persisting. Tails diphycerca! or heterocercal; 

 pectoral fins scaled at base; broad gular plates beneath jaws in place of branchin- 

 g-trails; skeleton well ossified. Polyptems and Calamoichthys, Africa. Probably 

 ancestral to the Amphibia. 



Order II. Chondrostei. 



These resemble sharks externally in heterocercal tail, spiracle, ventral 

 position of mouth; internally in cartilaginous skull and (except Polyodon) in 

 pterygoquadrate serving as upper jaw. Vertebral column primitive, centra 

 lacking, neural and haemal arches and intercalaria resting on notochordal sheath 



FIG. 558. Acipenser sturij* common sturgeon (after Goode). 



(fig. 512). ACIPENSERID.E, large bony dermal plates. Acipenser,* sturgeon. 

 POLYODONTHXS:, naked skin and long paddle-like snout, toothed maxillaries 

 present. Polyodon,* paddle fish. 



Order III. Holostei. 



Skull is ossified as in teleosts; maxillary and premaxillary bones present, 

 pterygoquadrates reduced and not meeting in front, mouth terminal. Body 

 with ganoid or cycloid scales. The living forms (the group appears in the trias) 

 have no spiracle and diphy- or homocercal tails. LEPIDOSTEID^. Scales 

 rhomboid, branchiostegal rays present, vertebrae opisthocoele. Lepidosteus,* 

 fjarpiki-. AMIID.E, distinctly teleostean in appearance with cycloid scales, 

 amphiccelous vertebrae, and heart with reduced conus (fig. 553, B). Amia,* 

 bow fin. 



Sub Class III. Telcostci. 



The teleosts owe their name to the extensive ossification of the skeleton, 

 which consists, in the trunk, of amphiccelous vertebras bearing large 

 ribs (p. 454), and in front a skull with numerous primary and secondary 

 bones, already enumerated (p. 495, fig. 547). Maxillaries and 

 premaxillaries are present, but these are frequently without 

 teeth, since other bones of the mouth (vomers, palatines, para- 



