50S CIIORDATA. 



Icits having opisthocrele vertcbra\ There are no characters in all ganoids 

 ^vhich occur onlv in the group. The ganoid scales are often absent, since 

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

 (Polvodoii) has almost no dermal skeleton, and Aviia has cycloid scales. 

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

 lying shingle-like in front of the tins (I'lg. io,B), but these are frequently 

 absent, e.g., in Ainia and Polyplcnis (fig. lo, .1 and C). The group is 

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



Order I. Crossopterygii. 



Larijelv extinct, two genera persisting. Tails diphycerca! or hctcroccrcal; 

 pectoral tins scaled at base; broad t^ii/ar />!atcs beneath jaws in ]ilace of branchio- 

 stesals; skeleton well ossilied. Polvplcnis and Cjla})ioiihlhys, Africa. Probably 

 ancestral to the Amphibia. 



Order II. Chondrostei. 



These resemble sharks externally in hcterocercal tail, spiracle, ventral 

 position of mouth; internallv in cartilaginous skull and (excejit Polyodoii) in 

 ptervgoquadrate serving as upper jaw. Vertebral column primitive, centra 

 lacking, neural and ha;mal arches and inlercalaria resting on notochordal sheath 



Fig. ssS. — .t(.7'/'(';/^"dT 5//(r/e,* common sturgeon l,a{Ler Cioodc). 



(fig. =;i2). AciPEXSERiD.E, large bony dermal plates. Aci/>ciiser* sturgeon. 

 PoLYODOXTiD.E, naked skin and long paddledike snout, toothed ma.xillaries 

 present. PolyoJoii* paddle tish. 



Order III. Holostei. 



Skull is ossilied as in telcosts; maxillary and prcmaxillarv 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 iliphy- or homocercal tails. Lepidosthid.e. Scales 

 rhomboid, branchiostcijai rays present, ve'-tebne opisthocade. Lcpidoslciis* 

 ,t;arpike. .Amiid.e, distinctly teleostcan in appearance with cycloid scales, 

 amphiccclous vertelira\ and heart with reduced conus (tig. 553, B). Th;/i7,* 

 bow tin. 



Sub Class III. Tclcosld. 



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

 which consists, in the trunk, of amphicadous vertebra- bearing large 

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

 bones, already enumerated (p. 4Q5, fig. 547). Maxillaries and 

 premaxillaries are present, liut these are frequently without 

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



