728 MR. c. TATE REGAN ON [June 19, 



from a study of the development of Selachians. The memoirs of 

 both authors were so complete and so lucidly written that no one 

 who carefully studies them can come to any conclusion other than 

 that the writers had proved their case, and that the theory of 

 Gegenbaur, that the paired fins and their girdles were derived 

 from posterior branchial arches and their rays, had been absolutely 

 and finally disposed of. 



However, this latter theory still continues to be put forward 

 by the Gegenbaurian school, and raost writers of text-books seem 

 to consider it of equal importance with the Thacher-Balfour 

 hypothesis. Some authors who accept the theory of the similar 

 origin of the median and paired fins have shown themselves to 

 be unacquainted with the facts of comparative anatomy and 

 embryology on which it is based, and have consequently failed in 

 the attempt to apply it to the elucidation of the relationships of 

 the various groups of fishes, whilst the writer of a recent memoir 

 (Kerr, No. 32) has so little understood Balfour's observations as 

 to ofier an absolutely impossible explanation of them. 



Thacher examined the median and paired fins in a number of 

 Selachian and Chondrostean Fishes. According to him, in the 

 former group the dorsal and anal fins are supported by a series of 

 cartilaginous rods, each usually composed of 3 segments [which 

 miay be called basals (within the body-wall), radials (in the 

 muscular lobe of the fin), and marginals (the distal segments)], 

 sometimes of 2 only, sometimes of more. Conci-escence of adjacent 

 segments frequently occurs. Each cartilaginous rod has on each 

 side a special muscle, separated from its fellows by the fibrous 

 sheets which run from between the rods to the integument. 

 Each muscle develops a flat tendon which lies parallel to the 

 surface of the fin and inserts itself in the fascia covering the 

 exterior of the cartilaginous rods and the proximal ends of the 

 horny fibres. 



Comparison of the dorsal and pelvic fins in Mustelus canis showed 

 Thacher that they were closely similar. He found that in the 

 pelvic fin the horny fibres, the muscles and their tendons, and the 

 cartilaginous supports were of the same structure and stood in the 

 same relation to each other as in the case of the dorsal fin. The 

 difference consisted only in that the series of basal segments of 

 the supporting cartilages in the dorsal fin remained separate, but 

 were represented in the pelvic fin by 2 basal cartilages — a shorter 

 anterior piece, the pelvis, vmited to its fellow in the median line, 

 and a longer posterior piece, the basipterygium. He considered 

 that the resemblance of the pelvic fin to the dorsal was at least as 

 close as to the pectoral, and that the formation of pelvis and 

 basipterygium was due to concrescence of the basal segments of 

 the cartilaginous supports, a process of common occurrence in the 

 unpaired fins. 



In the more specialised pectoral fin the pectoral arch was 

 compared to the pelvis, and the metapterygium to the basi- 

 pterygium, whilst the propterygium and mesopterygium were 



