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Show 1887.] PAIRED FINS OF CERATODUS. 11 specimen first described (fig. ]), and its relationships to the basal mesomere were the less definite of the two. It was here segmented into two pieces, while it was much more intimately connected with the two adjacent parameres than was the case in the former example. These two fins (figs. 1 and 3) stand alone, among those which I have examined, with respect to the great increase in number of the parameres of the postaxial lobe, and that also bears the cartilage now in question. If, as Haswell suggests (15, p. 8), this duplication of rays is reversionary to a " a pre-existing condition in which the fin-skeleton consisted of branching, jointed, cartilaginous elements," the only conclusion which seems to m e justifiable is that the appearance of this new element amounts to that of the reappearance of one which has been lost. Haswell has described an individual (15, figs. 6 and 7) in which the cartilage in question appears to have been present on both sides ; and it is instructive to remark that in both fins the rays of the postaxial series were, as with my specimens, the more numerous. I have already stated that in the fin described at the outset (fig. 1), the whole skeleton was, with the exception of the bar in question and the basal mesomere, very slight and leaflike. This simplification of structure, so suggestive of the reversion claimed by Haswell, is seen in the basal mesomere itself. That was (fig. 1, m.p.) much thinner and more flattened than is usual, and it bore but one processus muscularis (tb.) instead of the three described by Davidoff (cf. 7, pl. 8). All the foregoing facts point to the conclusion that the newly described cartilage exists only in fins whose postaxial rays remain little modified. There is, therefore, good reason to regard it, let its homology prove to be what it may, as atavistic. It has disappeared in the normal fin, under a confluence of the parameres of its own side, and a consequent thickening of the postaxial fin-lobe. III. The Pectoral member of Ceratodus compared with the Pelvic one of the same and the Pectoral one of the Plagiostomes. Haswell, reviewing (15, p. 5) the well-known observations and hypotheses of Balfour (1), Thacher (24, 25), and others, which led them to dissent from the interpretations of Gegenbaur and Huxley, says that they, together with the facts which he brings forward, seem to place it beyond a doubt that the limb of Ceratodus, " so far from representing a primitive and generalized type, is, as, indeed, we should expect from various other points in the organization of the animal, in reality highly specialized, and to be regarded as derivable from such simple limb-skeletons as those of the Selachii." In this he was anticipated by Balfour (I) whom he quotes. Balfour wrote (p. 669), when criticizing Huxley's position, the leading tenets of which he supported so far as the identification of the chief constituents of the fin-skeleton go, " I should be much more inclined to hold that the fin of Ceratodus has been derived from a fin like that of the Elasmobranchs, by a series of steps similar to those which Huxley supposes to have led to the |
