1871.] EGERTON LIASSIC CHIM^ROID FISH. 277 



be necessary to allude shortly to the anatomical characters of the 

 dorsal fin in the recent spinigerous Chondropterygians. Through the 

 kind assistance of Dr. Giinther, of the British Museum (whose ex- 

 tensive 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 Chimcera. In 

 the first-named genus, and also in Cestracion and Centrina, the 

 dorsal fin is supported by a broad cartilage imbedded in the mus- 

 cular 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 con- 

 sequence 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 Chi- 

 mcera, specimens of both which genera have been ably dissected 

 for me by Dr. Giinther. 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 Chimceridce 

 had a jointed dorsal spine, the Spinaddce and Cestraciontidce 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 afiinities of 

 the Chimseroid fishes. In the fossil under description the arrange- 

 ment of these parts corresponds in all material points with the 

 recent Chimseroids. The dorsal spine, however, was a more for- 

 midable 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 orna- 

 mented with fine tubercles arranged in longitudinal lines, and dimi- 

 nishing in size from the base of the spine upwards. Both these 

 and the spines consist of a hard lustrous ganoine, similar in ap- 

 pearance 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 



