DIPNOI. 249 



early development, the order has been said to^be intermediate 

 between the Fishes and Amphibia. By the form of the teeth 

 and the autostylic character of the skull, they resemble the 

 Holocephali ; while in the structure of the paired fins, the scales, 

 and, if we may judge by Polypterus, the form and function of 

 the air bladder, they approach the Crossopterygian Ganoids. 

 By their whole organisation they are essentially piscine, and in 

 many features of their structure, e.g. the nature of their endo- 

 skeleton and the structure of their vertebral column they 

 resemble the lowest fishes. In short they constitute an order 

 of fishes which, while showing affinity to most of the other 

 orders, are peculiar in possessing features which are not found 

 in other fishes but which are found in Amphibia. This 

 Amphibian tint, while not for one moment justifying an assign- 

 ment of the order to the Amphibia, or even to an intermediate 

 position between that class and Pisces, may by enlarging our 

 conception of the range of piscine structure be of service to us 

 in our speculative treatment of the numerous extinct forms 

 which, always imperfectly preserved, are in many cases most 

 difficult of interpretation. 



The gill slits are covered by an opercular fold. There are no 

 eyelids. The nasal sacs possess as in Teleosteans and Ganoids 

 two openings, but one of these is into the mouth on each side of 

 the vomerine teeth, the other on the under side of the head 

 close to the mouth. 



The tail is diphycercal, and the pelvic fins are inserted a little 

 in front of the anus. There are in Ceratodus two abdominal 

 pores leading into the body cavity, one on each side of, but a 

 little behind, the anus. In Lepidosiren abdominal pores are 

 not found ; in Protopterus it is said that there are two which 

 join to open by a single pore just in front of the anus within the 

 cloacal sphincter. 



The scales lie in pockets of the dermis and resemble in structure 

 the cycloid scales of Teleosteans. The superficial layer is without 

 bone-cells, which however are found in the deeper fibrous layer. 

 There is no ganoin. 



In the vertebral column the notochord persists and is uncon- 

 stricted, the sheath, which is invaded by cartilage, does not 

 segment or ossify ; there are dorsal and ventral arches which do 

 not meet laterally on the notochordal sheath (except sometimes 



