SKELETON OF STURGEONS. 75 



Polyodon), and the extinct the Cliondrosteidce, Palcconiscidce, 

 and (according to Traquair) Platysomidce. 



Their spinal column does not differ essentially from that 

 of the Dipnoi. Segmentation is represented only as far as the 

 neural and haemal elements are concerned. All are eminently 

 heterocercal. Eibs are present in most, but replaced by liga- 

 ments in Polyodon. 



The primordial cranium of the Sturgeons consists of per- 

 sistent cartilage without ossifications in its substance, but 

 superficial bones are still more developed and specialised than 

 in the Dipnoi ; so it is, at least, in the true Sturgeons, but less 

 so in Polyodon (Fig. 37). The upper and lateral parts of the 

 skull are covered by well-developed memhrane hones, which. 



pel 



Fig. 37.— Skull of Polyodon (after Traquair). 



n, Nasal cavity ; sq, squamosal ; mh, hyomandibular ; sy, symplectic ; pa, 

 palato-pterygoid ; m, Meckelian cartilage ; mx, maxillary ; d, dentary ; h, 

 hyoid ; op, opercle ; ir, branchiostegal ; s.cl, supra-clavicular; p.cl, post- 

 clavicular; cl, clavicle; i.cl, infra-clavicular. 



from this sub-order, upwards in the series, will be found to exist 

 throughout the remaining forms of fishes. They are bones, 

 the origin of which is not in cartilage but in membranous 

 connective tissue. The lower surface of the skull is covered 

 by an extremely large basal bone, which extends from the 

 vomerine region on to the anterior part of the spinal column. 

 The nasal excavation in the skull is rather lateral than inferior. 

 The ethmoidal region is generally much produced, forming 



