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Show 1876.] PROF. T. H. HUXLEY ON CERATODU8 FORSTERI. 49 and the skeletal elements on the dorsal side of the axis of the answer to those on the ventral side of the axis in the amphibian limb. As Dr. Gunther has observed, the contour of the fin in Ceratodus is somewhat like that of a sickle. The praeaxial edge is convex forwards and rather thicker, especially at its proximal end, than the posterior edge, which is concave backwards in its distal and convex in its proximal half. The apex of the fin is slender and recurved. A rounded and narrowed neck unites the limb with the trunk. Thus the limb, as a whole, is essentially unsymmetrical when its postaxial and praeaxial halves are compared. A corresponding asymmetry is strikingly obvious in the skeleton when it is prepared by removing the integument and muscles of the dorsal face, while the undisturbed condition of the parts is preserved by leaving the ventral integument and muscles untouched (fig. 10). It will be seen that, on the praeaxial side (Pr.a), each of the subquadrate segments of the median part of the skeleton, except the first and the terminal segments, gives attachment by its distal angle to a single jointed ray. The proximal or first ray (R) is much stouter than any of those which succeed it; and all take a direction approximately parallel to one another, their long axes forming an acute angle with that of the series of median segments. In the distal portion of the fin, the postaxial rays have a similar arrangement, and are only more slender than the praeaxial rays. But the second segment bears no fewer than five rays. Of these, the proximal, which is shortest and slenderest, stands out at right angles to the axis of the series of median segments ; while the others are gradually inclined at a less and less angle to it. The third segment and the fourth each carry two postaxial rays; the rest have but one. Dr. Giinther's figures show that, in his specimen also, the fourth and the third segments each bore two postaxial rays; but there are only four attached to the second segment, and all these are represented as if they had nearly the same inclination to the axis of the fin as the praeaxial rays. To dwell so strongly upon these minutiae may seem to be making a great deal of a very small matter; but its importance becomes manifest when the fin of Ceratodus is compared with that of other fishes. In m y " Preliminary Essay on the systematic arrangement of the Fishes of the Devonian epoch"*, I made use of the term "Crosso-pterygian" to express a peculiarity which is very strikingly manifest in the fishes to which I applied it, the fin-rays of the paired fins beino-disposed, like a fringe, round an oval, or elongated, central space covered with scales. The Crossopterygii, however, were not defined by this character alone ; and hence the fact that truly fringed fins are found beyond the limits of that group does not interfere with its perfectly natural character. In strictness, all fishes which possess paired fins are Crossopterygian in so far as the fin-rays always fringe the * Memoirs of the Geological Survey of the United Kingdom, decade PROC. ZOOL. Soc-1876, No. IV. 4 |
