OCR Text |
Show 10 PROF. G. B. HOWES ON THE SKELETON AND [Jan. 18, regard it as an instance of atavism, and so pointing back to a preexisting condition in which the fin-skeleton consisted of branching jointed cartilaginous elements supporting a cutaneous expansion considerably broader than that of the 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 morphologically double, and the longitudinal 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 Seitenstrahlen der dorsalen Halfte der einen Flosse entsprechen derjenigen der ventralen Halfte 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 supporting 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 the occasional bifurcation of a linear series of postaxial parameres, such as is represented in 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 inquired 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 |