10 PROF, G. B. HOWES ON THE SKELETON AND [Jan. 18, 



regard it as an instance of atavism, and so pointing back to a pre- 

 existing condition in which the fin-skeleton consisted of branching 

 jointed cartilaginous elements supporting a cutaneous expansion 

 considerably broader than that of tlie fin of the living Ceratodus 

 forsteri." If, as therein suggested, the typical paramere has 

 arisen from a confluence of branching-elements, such as exist 

 to-day among some Elasmobranchs, and if it be that the meso- 

 meres have been formed by the fusion of the basal ends of the 

 parameres as they now stand, each mesomere would be morpho- 

 logically double, and the longitvidinal cleavage of this axis would 

 thereby receive an intelligible interpretation. I am doubtful as to 

 the probability of such a process having been involved, but, in the 

 absence of any data upon the development of the fin, I put forward 

 the suggestion as a possible means of accounting for the apparent 

 irregularity. In support of this conception of the origin of the 

 parameres, it may be stated that their reduction in number is 

 proportionate to the thickening of the fin border. Schneider says 

 (23, p. 521) that "die Seitenstrablen der dorsalen Hiilfte der einen 

 Flosse entsprechen derjenigen der ventralen Hiilfte der anderen." I 

 find, however, that when (as in the left-hand fin of fig. 2 — right-hand 

 one of the drawing) that lobe which is generally thickened remains 

 thin, its supporting rays are more numerous and of smaller calibre 

 than usual. When, on the other hand, as was also the case in the 

 fin represented, the usually thin lobe becomes thickened, its support- 

 ing rays get less numerous in proportion as they become more 

 powerful. Stated otherwise, these facts go far to prove that the 

 thickening of one or other of the fin- borders is mainly due to 

 confluence and subsequent increase in calibre of the parameres. 

 Suggestive, indeed, in view of all this is tlie occasional bifurcation 

 of a linear series of postaxial parameres, such as is represented in 

 fig. 7. 



Perusal of the foregoing pages will show conclusively that 

 Haswell's " branching " fin is, when compared with those of a number 

 of other individuals, little, if at all abnormal. There yet remains for 

 consideration that cartilage {r, fig. 1) which, as I have stated, is 

 connected with the proximal mesomere ; and it has now to be incjuired 

 if a representative thereof is forthcoming in a more typical fin. 

 After long searching I found an unmistakable representative of 

 it, and that in none other than the left fin of the remarkable pair 

 represented in fig. 2. Fig. 3 is a drawing of the upper third of 

 the same. The postaxial lobe was supported, as has been already 

 stated, by a series of delicate parameres, of which there were two 

 to each of the ray-bearing mesomeres figured, with the exception 

 of the first and third {cf. fig. 2). The preaxial lobe was, contrary 

 to the general rule, supported by a series of larger and more powerful 

 parameres ; of these there was one to each of the above-named 

 segments, with the exception of the first. None of the parameres 

 showed the slightest trace of branching. 



Proximally to the postaxial rays there lay the cartilage, r, of 

 fig. 3. This element was relatively far smaller than was that of the 



