﻿ACA1TTH0DII. 



proximally in a smaller, abruptly truncated expansion. This 

 element has a thick, smooth, calcined surface, and its long axis 



Fig. 2. 



Skeleton of pectoral fin of Acanthodes bronni, Ag. 

 &, basal cartilage ; r, fibrous rays ; s, anterior spine. 



seems to have been originally more or less vertical, while there is no 

 evidence of a connection with its fellow of the opposite side. By 

 Huxley, Kner, Egerton, and others, this has been regarded as a 

 representative of the pectoral arch ; and the interpretation may 

 appear at first sight justified by the relatively large size of the car- 

 tilage in some Diplacanth genera. To the present writer, however, 

 the element in question seems to pertain to the basipterygium ; for 

 it exhibits the same relative size and position as the basal cartilage 

 in the spinous dorsal fins of several sharks ; and in a well-preserved 

 example of another Acanthodian, Pareocus falcatus (No. P. 130, 

 p. 35), a much larger, expanded, triangular element, more delicate, 

 apparently meets its fellow in the middle line, and occupies the 

 position with respect to the spine that a pectoral arch might be 

 expected to hold. No other cartilage is recognizable, but at a short 

 distance below the supposed basipterygium there occurs a close series 

 of short, fine dermal fin-rays (r), sometimes appearing as the fringe 

 of a short obtuse lobe ; and it may be that these mark the precise 

 limit of the endoskeletal part of the appendage. 



As often shown in the type species 1 , the anterior part of the 

 lower lobe of the caudal fin is supported by a series of long, stout, 

 basal cartilages (? haemal spines), each apposed to a short haemal 

 arch, but distinctly separated from the latter. The Acanthodian 

 caudal fin thus presents a resemblance to the corresponding fin of 

 certain Selachians, e. g. Mustelus antarcticus 2 . 



1 Kner, Sitzungsb. k. k. Akad. Wiss. Wien, math.-naturw. Cl. vol. Mi. pt. i. 

 pi. v. fig. 2, pi. vii. fig. 1. 



2 Mivart, Trans. Zool. Soc. vol. x. (1879) p. 441, pi. lxxiv. fig. 6. 



