1906.] SELACHIAX FISHES. 731 



angles to the longitudinal bar, or basipterygium, which is 

 continuous in front with the fin-girdle. Fui-ther changes in the 

 pelvic fin consist chiefly in the segmentation of basipterygium 

 from pelvis and of the radials fi'om both. The pectoral arch 

 develops as a vertical bar of cartilage at the front border of the 

 rudimentary fin, and externally to the muscle-plates * ; the first 

 part to be formed is that in the neighbourhood of the basi- 

 pterygium, the dorsal and ventral prolongations being subsequent 

 outgrowths. Changes similar to those described in the pehdc 

 fin occur in the pectoral, but, in addition, the basipterygium 

 (metapterygium) rotates outwards and comes to form the posterior 

 border of the skeleton of the fin. 



Balfoiu-'s conclusions are that the homology of the pectoral 

 metapteiygium with the pelvic basipterygium is established, 

 and its primitive position is shown to be within the body- 

 wall and parallel to the long axis of the body. The pelvic fin 

 represents a stage in the evolution of the pectoral fin, and what 

 Gegenbaur conceived to be the primitive axis of the biserial fin is 

 demonstrated to be really the base, so that post -axial rays must 

 be secondary. The mode of development of the fin-giixlles is in 

 favour of the h}^othesis that they are outgTOwths of the basi- 

 pterygia, and the latter may well have been formed from the 

 coalescence of the originally separate basal segments of the 

 supporting cartilages, since in the median fins also these are 

 segTnented from continuous laminae. On the other hand, it is 

 difficult to see how a limb formed on the tj-pe of the embryonic 

 limb of Elasmobranchs could be derived from a viscei'al arch with 

 its branchial rays. 



The fact that the theory of the similar origin of the median 

 and paired fins was put forward on the gi-ound of their similar 

 structui'e and development in the living Selachians and Ohon- 

 drostean Fishes cannot be too strongly emphasised. To consider 

 the Euselachii and Chondrostei as respectively derived from the 

 Ichthyotomi and Crossopterygii, in which the paired fins are of a 

 more specialised type, is to ignore the evidence on which the 

 theory i^ests. 



The order Pleuropterygii includes the Devonian Cladoselachus, 

 which had broad-based paired fins, the pemcs without fusion of 

 the basaiia, the pectorals scarcely more advanced in structure t. 

 Cladodus and Symmorium, of which only the pectoral fins are 

 known, may be placed in this order provisionally, but are perhaps 

 transitional to the Ichthyotomi and Euselachii. The anterior 



* Balfour has evidentlj' italicised this phrase, because he has shown elsewhere 

 that the branchial bars are developed in the deeper parts of the mesoblast which 

 constitutes the primitive branchial arches, and on the inner side of the section of the 

 bodj'-cavitj' primitivelj' present in the arches. 



t The stronglj' heterocercal caudal tin described by Dean shows that Clado- 

 selaclius was more or less pelagic, and lends no support to the view that the broad- 

 based paired tins of this fish were a special adaptation to bottom living. 



