54 PROF. T. H. HUXLEY ON CERATODUS FORSTERI. [Jan. 4, 



Gegenbaur has most ingeniously suggested that the pectoral arch, 

 with its limb, would correspond with a branchial arch and its rays. 



It will be observed that the view of the special homologies of the 

 elements of the skeletons of the fins of fishes which I have ventured 

 to put forth differs, fundamentally, both from that suggested by Dr. 

 Giinther and from that advanced by Gegenbaur, either in its original 

 form or as he has modified it subsequently to the discovery of 

 Ceratodus. 



The former says (I. c. p. 533) : — " When I designated the arrange- 

 ment of the parts of this pectoral skeleton unique, I did not mean to 

 convey the idea that no homological relation could be pointed out 

 between the parts of the pectoral skeleton of Ceratodus and that of 

 other fishes. It is quite evident that we have here a further deve- 

 lopment of the simple pectoral axis of Lepidosiren in the direction 

 towards the Plagiostomes. The pectoral skeleton of Lepidosiren 

 paradoxa consists merely of the central series of cartilages of Cera- 

 todus ; there is no fin-like expansion of the skin of the pectoral limb, 

 which is a simple tapering filament. In Lepidosiren annectens this 

 pectoral filament is bordered by an expansion of the skin along its 

 lower edge ; and even minute fin-rays are imbedded in each lamina of 

 the fold ; in order to support this low, one-sided, rayed fringe, very 

 small, single-jointed cartilages are added to the axis*. The fin is 

 still more developed iu Ceratodus : it has become a broad, scythe- 

 shaped paddle, dilated by a fold of the skin, with two layers of fin- 

 rays surrounding it in its entire circumference ; therefore supporting 

 cartilaginous branches are added on both sides of the axis ; and most 

 of the branches are composed of several joints, in order to reach the 

 more distant parts which require the support." 



This is the exact converse of the view of the relations of Lepido- 

 siren and Ceratodus which, in agreement with Gegenbaur, I am 

 disposed to take. The fin of the former appears to me to be a 

 reduced and metamorphosed state of the more primitive condition 

 retained in Ceratodus. 



Dr. Giinther goes on to say that "the arrangement of the limb-ske- 

 leton of Ceratodus is foreshadowed in the pectoral fin of Acipenser." 

 On the contrary, in my judgment, the pectoral fin of Acipenser has 

 been derived by much modification from a Ceratodus-X\k& type. 



In referring to those points in which I venture to dissent from 

 Professor Gegenbaur's interpretation, I cannot refrain from express- 

 ing my sense of the very great value of his investigations into the 

 morphology of vertebrate limbs, and my grateful indebtedness to 

 th» rich fund of new facts and new ideas which they contain. 

 However, I found myself unable fully to accept his theory of the 

 fish's fin and the vertebrate limb generally, in its original form ; and 

 I expressed my hesitation and its grounds in the German version of 

 my 'Manual of the Vertebrata'f . Gegenbaur's later view is con- 



* Four or fire of these ray-bearers are obliquely attached to each joint of the 

 axis (Peters, MiiUcr's ' Archiv,' 1845, Taf. 2. fig. 2). 



t ' Handbuch der Anatomie der Wirbeltbiere,' iiberselzt von Dr. F. Ratzel 

 (Breslau, 1873), pp. 34, 35. 



