SKELETON OF POLYPTEEUS. 77 



corner of a basal cartilage {a) a short axis (5) is joined, 

 •which on its outer side bears a few branches {d) only, the 

 remaining branches (c) being fixed to the basal cartilage. 

 The dermal fin-rays are opposed to the extremities of the 

 branches, as in the Dipnoi. 



The puhic consists of a paired cartilage, to wliich tarsal 

 pieces supporting the iin-rays are attached. 



The other living Ganoid fishes have the spinous column 

 entirely or nearly entirely ossified, and have been comprised 

 under the common name Holostei. However, they form three 

 very distinct types; several attempts have been made to 

 coordinate with them the fossil forms, but this task is beset 

 with extreme difficulties, and its solution hitherto has not 

 proved to be satisfactory. 



The Polypteroidei have their spinous column formed by dis- 

 tinct osseous amphicmlous verteb^'ce, that is, vertebrse with 

 concave anterior and posterior surfaces. It is nearly diphy- 

 cercal ; a slight degree of heterocercy obtains, inasmuch as 

 the last vertebra is succeeded by a very thin cartilaginous 

 filament which penetrates between the halves of one of the 

 middle rays of the terminal fin. The rays above this cartila- 

 ginous filament are articulated to interneurals, those below 

 lack interhsemals, and are attached either to the hsemals or 

 vertebral centres. The neural arches, though ossified, do not 

 coalesce with the centrum, and form one canal only, for the 

 myelon. There are no intermediate elements between the 

 neural spines. Interneurals developed, but simple, articulat- 

 ing with the dermoneurals. The abdominal vertebrae have 

 parapophyses developed with epipleural spines. Only the 

 caudal vertebrae have haemal spines, which, like the inter- 

 hsemals, agree in every essential respect with the opposite 

 neurals. Rihs are inserted, not on the parapophyses, but on 

 the centre, immediately below the parapophyses. 



The skull of Polypterus (Fig. 39) shows a great advance 



