FI]S"S OF aANOIDS AND TELEOSTS. 533 



I would suggest, therefore, tlie use of tbe term " radial element " 

 as tlie unit of the series of skeletal fiu-supportiug bones, or 

 cartilages, in both the mesial and paired fins ; and in those 

 instances in which such elements undergo segmentation, the terms 

 proximal, mesial, or distal segments may be adopted. 



The various species referred to in the descriptive section of 

 this paper are those enumerated by Dr. Giiuther in the British 

 Museum Catalogue of Fishes (1st ed.), and for this reason the 

 authorities for the specific names have been omitted in the text. 



In most instances in the description of the radial elements of 

 different species the number of these elements has been given, 

 but as these structures are liable to some slight individual varia- 

 tion in the same species, the number mentioned must be taken as 

 applying ouly to the particular specimen examined. 



II. DESCRIPTIVE. 



ELASMOBEANCHII. 



The dorsal and anal fins, but more particularly the dorsal fins, 

 have been so fully and carefully described by Thacker [5] and 

 Mivart [6] , that it is unnecessary to do more than direct atten- 

 tion to a few of their results for the sake of comparison with 

 other types. In the majority of the species described and figured 

 by Mivart {I. c.) the radial elements are cartilaginous, rod-like 

 structures, generally of fairly uniform thickness throughout 

 their length, and iisually divided into proximal, mesial, and distal 

 segments. The individual segments vary in length, and, in 

 different species, each may in turn become the longest. The 

 various radial elements in each fin may afford mutual support to 

 one another, and gain in strength, through their arrangement iu 

 close parallel relations throughout their entire length, but occa- 

 sionally they may separate slightly from one another, either 

 proximally or distally, or even at both extremities. In no 

 instance is there any definite articulation between particular 

 segments of contiguous radial elements. The central, or apj^roxi- 

 mately, central, radial elements in either fin are usually the 

 longest, but almost invariably the most anterior and posterior of 

 the series undergo a reduction iu length and also lose one or 

 more of their constituent segments. 



From this general type of fin-structure the more important 

 deviations in particular genera are brought about by (a) the 

 more or less extensive longitudinal concrescence of the proximal 



