4 STKUCTTJEE OF THE PAIEED FIXS OE CERATODrS, 



It will thus be seen that both Griinther and Huxley, while their 

 descriptions differ in various minor points connected with the 

 arrangement of the cartilages, agree in regarding the fin of 

 Ceratodus as essentially uniaxial, i. e. consisting of a single jointed 

 axis, with the preaxial and postaxial sides of which a series of 

 jointed cartilaginous rays are connected. 



A similar, though simpler, arrangement had already been 

 described as characterising the fin of Lefidosiren, and before the 

 discovery of Ceratodios, Gregenbaur had come to the conclusion 

 from theoretical considerations that the fundamental type of the 

 Vertebrate limb very much resembled the fin of Lepidosiren. 

 This primitive limb he designated the arclilpterygium, and in 

 connection with it he remarks in the second edition of his 

 " Grrundriss der Vergleichenden Anatomic " (F. Jeffrey Bell's 

 Transl. 1878, p. 473) — " When simplest this (the skeleton of the 

 free part of the limb), is made up of cartilaginous rods (rays), 

 wdiich differ in their size, segmentation and relation to one another. 

 One of these rays is larger than the rest and has a number of 

 other rays attached to its sides. I have given the name of 

 Archipterygium to the ground form of the skeleton which extends 

 from the limb-bearing girdle into the free appendage. The 

 primary ray is the stem of this archipterygium, the character of 

 which enables us to follow out the lines of development of the 

 skeleton of the appendage. Cartilaginous arches beset with rays 

 form the branchial skeleton. The form of skeleton of the 

 appendages may be compared with them ; and we are led to the 

 conclusion that it is possible that they may have been derived 

 from such forms. In the branchial skeleton of the Selachii the 

 cartilaginous bars are beset with simple rays. In many, a 

 median one is developed to a greater size. As the surrounding 

 rays become smaller, and approach the larger one, we get an 

 intermediate step towards that arrangement in which the larger 

 median ray carries a few smaller ones. This differentiation of 

 one ray, which is thereby raised to a higher grade, may be con- 



