46 PKOF. T. H. HUXLEY ON CKRATODUS FORSTERI. [Jan. 4, 



III. The Pectoral Limb. 



Ill the memoir cited. Dr. Giiuther describes the limbs of Ceratoilus 

 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 bro»d 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 hmb, 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 Plagiostomcs *. 

 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 

 ])osterior 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 metapterygium of Gegenbaur. 



