SKELETON OF DIPNOI. 71 



the dorsal fin is armed articulates with a neural apophysis, 

 and is not immovably attached to it, as in the Sharks. The 

 pubic consists of two lateral halves, with a short, rounded, 

 tarsal cartilage. 



The skeleton of the Ganoid Fishes offers extreme varia- 

 tions with regard to the degree in which ossifications replace 

 the primordial cartilage. Whilst some exhibit scarcely any 

 advance beyond the Plagiostomes with persistent cartilage, 

 others approach, as regards the development and specialisa- 

 tion of the several parts of their osseous framework, the 

 Teleosteans so closely that their Ganoid nature can be 

 demonstrated by, or inferred from, other considerations only. 

 All Ganoids possess a separate gill-cover.-^ 



The diversity in the development of the Ganoid skeleton 

 is well exemplified by the few representatives of the order in 

 the existing Fish-faima. Lowest in the scale (in this respect) 

 are those with a persistent notochord, and an autostylic skull, 

 that is, a skull without separate suspensorium — the fishes 

 constituting the sub-order Dipnoi, of which the existing repre- 

 sentatives are Lepidosiren, Protopterus, and Ceratodus, and the 

 extinct (as far as demonstrated at present) Dipterus, Chirodus 

 (and PhaneropUuron ?). In these fishes the notochord is 

 persistent, passing uninterruptedly into the cartilaginous base 

 of the skull. Only now and then a distinct vertical segment- 

 ation occurs in the caudal portion of the column, but it does 

 not extend to the notochord itself, but indicates only the 

 limits between the superadded apophyseal elements, each 

 neural being confluent with the opposite hsemal. Some Dipnoi 

 are diphy-, others hetero-cercal. 



^ The Ganoids formed at former epochs the largest and most important 

 order of fishes, many of the fossil forms being known from very imperfect 

 remains only. It is quite possible that not a few of the latter, in which 

 nothing whatever of the (probably very soft) endoskeleton has been preserved, 

 should have to be assigned to some other order lower in the scale of organisa- 

 tion than the Ganoids (for instance, the Cephalaspidce). 



