Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/413

 be necessary to allude shortly to the anatomical characters of the dorsal fin in the recent spinigerous Chondropterygians. Through the kind assistance of Dr. Gunther, of the British Museum (whose extensive knowledge of ichthyology is not surpassed by any living authority), I have been enabled to examine the structure of the dorsal fin in the recent Acanthias, Callorhynchus, and Chimoera. In the first-named genus, and also in Cestracion and Centrina, the dorsal fin is supported by a broad cartilage imbedded in the muscular tissue of the back of the fish. The anterior upper angle of this cartilage carries the dorsal spine, projecting into the cavity of the spine itself. The spine has its base planted in the muscular tissue, and is embraced by a fold of skin at the base of the fin. The internal portion of the spine is defined by the absence of the horny covering which characterizes the external parts. This feature is well seen in some of the large fossil Ichthyodorulites. In consequence of this arrangement the spines of these fishes had a very limited amount of freedom, probably not more than could be allowed by the elasticity of the integuments. The case is far otherwise as regards the dorsal spine of Callorhynchus and Chimoera, specimens of both which genera have been ably dissected for me by Dr. Gunther. Here the broad cartilage which supports the dorsal fin, and which carries the dorsal spine on its anterior margin, is external to the muscular tissue, and is enveloped only by dermal integument. The anterior process forming the core of the hollow spine has its lower extremity developed into a rounded articulating surface, which works in a cavity on the upper anterior edge of a strong cartilaginous plate extending upwards from the notochordal axis. A perfect joint is thus formed (1 d), enabling the spine to move freely in the vertical plane for nearly one-fourth of a circle, very much as the mast of a barge is lowered and raised before and after passing under a bridge. In short the Chimoeridoe had a jointed dorsal spine, the Spinacidoe and Cestraciontidoe a fixed one. I am not aware that this fact has been noticed before ; it is one of no small importance in considering the natural affinities of the Chimaeroid fishes. In the fossil under description the arrangement of these parts corresponds in all material points with the recent Chimaeroids. The dorsal spine, however, was a more formidable weapon. It measures six inches in length from the joint to the apex, by three lines in breadth. It is slightly recurved, and is armed with a single series of uncinate spines pointing upwards on the proximal, and a double series pointing downwards on the distal, margins of the spine. The remainder of the surface is ornamented with fine tubercles arranged in longitudinal lines, and diminishing in size from the base of the spine upwards. Both these and the spines consist of a hard lustrous ganoine, similar in appearance to that composing the tubercles on the rostrum. The articulating facet at the junction of the spine with the body is carried on the base of the cartilaginous core which occupies the cavity of the spine. It differs slightly from the recent analogue in being rather concave than convex, the corresponding articulating