STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 95 



ventral parts of the haemal arches of the tail, which, like 

 them, are in many instances (Ceratodus, Lepidosteus, Poly- 

 pterus, and to some extent in Amia) segmented off from the 

 basal parts of the haemal arches. 



In Teleostei the ribs have the same position and relations 

 as those in Ganoids and Dipnoi, but their serial homology 

 with the ventral parts of the hsemal processes of the tail is 

 often [e.g. the Salmon) obscured by the anterior hsemal 

 arches {i.e. those in the posterior part of the trunk) being 

 completed, not by the ribs, but by independent outgrowths 

 of the basal parts of the haemal processes. 



In Elasmobranchii a still further divergence from the 

 primitive arrangement is present. The ribs appear to have 

 passed outwards, along the intermuscular septa, into the 

 muscles; and are placed between the dorso-lateral and 

 ventro-lateral muscles (a change of position of the ribs of the 

 same nature is observable in Lepidosteus). This change of 

 position, combined probably with the secondary formation 

 of a certain number of anterior haemal arches, similar to 

 those in the Salmon, renders their serial homology with the 

 ventral parts of the hsemal processes of the tail far less clear 

 than in other types ; and further proof is required before 

 such homology can be considered as definitely established. 



Under the third heading the skeletal elements supporting 

 the fin-rays of the ventral lobe of the caudal fin of various 

 types of fishes are compared, and the following conclusions 

 are arrived at : 



(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 by indently developed interspinous 

 bones. 



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

 supported by haemal arches, may be used in deciding 

 whether apparently diphy cereal tail-fins are aborted or 

 primitive. 



Urogenital Organs — With reference to thecharacter of the 

 adult urogenital organs, the authors show that for the female 

 the descriptions ot Miiller and Hyrtl are substantially 

 accurate, but that Hyrtl^s description of the generative ducts 

 of the male is wholly incorrect. 



They find that in the male the semen is transported from 

 the testes by means of a series (40 — 50) of vasa effereutia, 

 supported by the mesorchium. In the neighbourhood of the 

 kidney these vasa unite into a longitudinal canal, from which 

 transverse trunks are given off, which become continuous 



