G A. VOIDS. 249 



Am in and Lcpidostcns are noticeable for the sucking disk de- 

 veloped on the front of the head ; the larva of the sturgeon has 

 balancer-like structures between the mouth and nose. 



ORDER I. CROSSOPTERYGII. 



Ganoids with diphy- or heterocercal tails ; pectoral fins with 

 a large basal portion covered with scales, the ventral fins usually 

 much like the pectorals, the former abdominal in position ; body 

 covered with rhombic or circular ganoid scales ; a pair of gular 

 (or 'jugal') plates in place of branchiostegals ; no fulcra; dor- 

 sal fins two, or a single one divided into many finlets. The 

 crossopterygians are largely extinct, but two genera persisting 

 to-day. The group first appears in the Devonian, 



FIG. 250. Diplurus longicaudatus, from Dean. A, position of the 

 calcified air-bladder. 



The CcELACANTHiDvfc (AcTixiSTiA) have unossified ce.ntra and cycloid 

 scales, two dorsal fins, diphycercal caudal, and ossified swim-bladder. 

 C&lacanthus, carboniferous of Europe and Ohio ; Diplurus, trias of New 

 Jersey. CYCLODIPTERINI (RHIPIDISTIA) have the vertebrae partially ossi- 



FIG. 251. Head of larval Polypterns, after Steindachner, from Dean. 

 G, external gill. 



fied ; tail heterocercal ; scales enamelled and rounded behind ; a thircT 

 gular plate between the other two. Holoptychius, Devonian of Europe and 

 America ; Onychodus and Eusthenopteron, Devonian of America. RHOM- 



