SiO STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 



(4) We believe that in Ccratodus we have been able to trace 

 a small number of the ventral fin-rays supported by haemal 

 arches only, but these rays are so short as not to extend so 

 far back as some of the rays attached to the interspinous elements 

 in front. These rays may probably be interpreted, like the more 

 or less corresponding rays in the tail of the Eel, as the last 

 remnant of a true caudal fin. 



The above considerations appear to us to shew with very 

 considerable probability that the true caudal fin of the Dipnoi 

 has become all but aborted like that of various Teleostei ; and 

 that the apparent caudal fin is formed by the anal and dorsal fins 

 meeting round the end of the stump of the tail. 



From the adult forms of Dipnoi we are, however, of opinion 

 that no conclusion can be drawn as to whether their ancestors 

 were provided with a diphycercal or a heterocercal form of 

 caudal fin. 



The general conclusions with reference to the tail-fin at which 

 we have arrived are the following : 



(1) The ventral lobe of the tail-fin of Pisces differs from the 

 other unpaired fins in the fact that its fin- rays are directly 

 supported by spinous processes of certain of the haemal arches 

 instead of independently developed interspinous bones. 



(2) The presence or absence of fin-rays in the tail-fin 

 supported by haemal arches may be used in deciding whether 

 apparently diphycercal tail-fins are aborted or primitive. 



EXCRETORY AND GENERATIVE ORGANS. 



I. Anatomy. 



The excretory organs of Lepidosteus have been described by 

 Miiller (No. 13) and Hyrtl (No. 11). These anatomists have 

 given a fairly adequate account of the generative ducts in the 

 female, and Hyrtl has also described the male generative ducts 

 and the kidney and its duct, but his description is contradicted 

 by our observations in some of the most fundamental points. 



In the female example of ioo'5 centims. which we dissected, 

 the kidney forms a paired gland, consisting of a narrow strip of 

 glandular matter placed on each side of the vertebral column, on 



