STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 809 



that, although there is a prolongation backwards of the verte- 

 bral axis beyond the last interspinous elements, composed it 

 would seem of the coalesced neural and haemal arches but 

 without the notochord, yet by far the majority of the fin-rays 

 which constitute the apparent caudal fin are supported by inter- 

 spinous elements. 



The grounds on which we hold that the tail of the Dipnoi is 

 to be regarded as a degenerate rather than primitive type of tail 

 are the following : 



(1) If it be granted that a diphycercal or protocercal form 

 of tail must have preceded a heterocercal form, it is also clear 

 that the ventral fin-rays of such a tail must have been supported, 

 as in Polypterus and Calamoicthys, by haemal arches, and not by 

 interspinous elements ; otherwise, a special ventral lobe, giving 

 a heterocercal character to the tail, and provided with fin-rays 

 supported only by haemal arches, could never have become 

 evolved from the protocercal tail-fin. Since the ventral fin-rays 

 of the tail of the Dipnoi are supported by interspinous elements 

 and not by haemal arches, this tail-fin cannot claim to have the 

 character of that primitive type of diphycercal or protocercal 

 tail from which the heterocercal tail must be supposed to have 

 been evolved. 



(2) Since the nearest allies of the Dipnoi are to be found in 

 Polypterus and the. Crossopterygidae of Huxley, and since in 

 these forms (as evinced by the structure of the tail-fin of Polyp- 

 terus, and the transitional type between a heterocercal and 

 diphycercal form of fin observable in fossil Crossopterygidae) the 

 ventral fin-rays of the caudal fin were clearly supported by 

 haemal arches and not by interspinous elements, it is rendered 

 highly probable that the absence of fin-rays so supported in the 

 Dipnoi is a result of degeneration of the posterior part of the 

 tail. 



[We use this argument without offering any opinion as to 

 whether the diphycercal character of the tail of many Crossop- 

 terygidae is primary or secondary.] 



(3) The argument just used is supported by the degenerate 

 and variable state of the end of the vertebral axis in the Dipnoi 

 a condition most easily explained by assuming that the terminal 

 part of the tail has become aborted. 



B. 52 



