CROSSOPTER YGI I . 



Order 2. CKOSSOPTERYon.* 



Vertebral column ossified, or unossi- 

 fied. Tail diphycercal or hetero-diphy- 

 C6rcal {upper lobe of caudal fin weaker 

 than the lower lobe, Fig. 103). Paired 

 fins with scaled basal lobe which is either 

 pointed or rounded. In the former 

 case it is unibasal, rachiostichous and 

 mesorachic, in the latter case, found only 

 in living forms, the pectoral fin is 

 tribasal and rhipidostichous. In place 

 of the branchiostegal rays are two large 

 jugular plates between the rami of the 

 lower jaw, near which there are in many 

 palaeozoic forms a number of smaller 

 lateral plates and a median anterior 

 plate. Ganoid scales, rhombic or 

 cycloidal, cover the whole body and 

 tail. Dorsal fin either two in number ; 

 or if single, long or multifid. Devonian 

 to present time. 



This order which was established by 

 Huxleyt in 1861, is mainly based on 

 the form of the fin, which is fringed 

 with dermotrichia on both sides. It 

 is difficult to state any other charac- 

 ters peculiar to the living and extinct 

 forms, unless it be the two jugular 

 plates between the rami of the lower 

 jaw. 



The following remarks apply to the 

 living Polypteridae : — 



The scales of Polypterus are very 

 similar to those of Lepidosteus (see 



171 



Fig. 97. — Clieirolepis trailli ; re- 

 stored (except margin of tins) 

 by Xraquair (from Smith- 

 Woodwara), quarter natural 

 size. 



p. 177). They are 



* Traquair, Cranial Osteology of Polypterus, Journ. Anat. and 

 Physiology, 4, 1871. Pollard, " Anatomy of Polypterus," Zool. Jahrb., 5, 

 1892. 



t Preliminary Essay upon the systematic arrangement of the fishes 

 of the Devonian Epoch, Mem. Geol. Survey United Kingdom, 1861. 



