46 PROF. T. H. HUXLEY ON CERATODUS FORSTERI. [Jail. 4, 



III. The Pectoral Limb. 

 In the memoir cited, Dr. Gmither describes the limbs of Ceratodus 

 and their skeleton as follows: — 



" The limbs consist of two pairs of paddles, similar in appearance 

 to the termination of the tail ; viz. a longitudinal axis, formed by 

 the endoskeleton and muscles and covered with scales, is surrounded 

 by a broad rayed fringe. These paddles are structurally identical 

 with the fins of Lepidosiren ; only the axis and also the fringe are 

 much dilated. The pectoral and ventral paddles taper to a fine point, 

 the former being longer than the head, the latter rather shorter. 

 The ventral paddles are inserted at a short distance in front of the 

 vent" (p. 515). 



" The paddle is joined to the scapular arch by an elongate, flattish, 

 slightly curved cartilage ; its proximal end has a glenoid cavity, 

 fitting into the humeral condyle ; the joint is simple, free, allowing 

 of a considerable amount of motion, its parts being held together 

 by a ligament fastened round its circumference. This is the only 

 true joint in the limb, all the other parts being fixed to one another 

 by connective tissue. I consider this cartilage to be the forearm ; a 

 horizontal section along its longitudinal axis does not show any pri- 

 mary division. The next following cartilage forms the base of the 

 paddle ; although externally it appears as a single flat, broad, short 

 piece, unevennesses of its surface indicate that several primary pieces 

 are coalesced in it. 



" I am confirmed in this view by a horizontal section, in which 

 the lines of the former divisions are preserved in the shape of tracts 

 of a white connective tissue. Three such divisions may be distin- 

 guished, corresponding to the three carpals of most Plagiostomes *. 

 If this determination is correct, then the antibrachial cartilage just 

 described is not represented in that order. 



" The remaining framework of the paddle shows an arrangement 

 unique among the Vertebrata. From the middle of the basal carti- 

 lage a series of about twenty-six subquadrangular pieces takes its 

 origin, forming a longitudinal axis along the middle of the paddle 

 to its extremity. The pieces become gradually smaller, and are 

 scarcely distinguishable towards the end of the paddle. On the two 

 posterior corners of each piece a branch is inserted, running ob- 

 liquely backwards towards the margin of the fin ; the branches of 

 the first eight or twelve pieces are three-jointed, the remainder two- 

 jointed, the last having no branch at all. Slight irregularities, such 

 as the origin of two branches from one side of a central piece, occur, 

 as also several four-jointed branches being inserted immediately on 

 the basal cartilage " (pp. 532-3). 



In general, this description suits the pectoral fins of the specimen I 

 have described very well. Mine, however, has only twenty median 

 cartilages. All but the very last bear lateral rays ; but towards the 

 distal end of the fin these become minute, and consist of a single 

 piece. Moreover the distal joints are much more slender, especially 



* Pro-, meso-, and nietaplcrvgium of Gogenbaur. 



