4 
ACAXTHODII. 
proximally in a smaller, abruptly truncated expansion. This 
element has a thick, smooth, calcified surface, and its long axis 
Fig. 2. 
Skeleton of pectoral fin of Acanthodes bronni , Ag. 
b, 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, Parexus fctlcatus (Ho. 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. Ho 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 x , 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. lvii. pt. i, 
pi. v. fig. 2, pi. vii. fig. 1. 
2 Mivart, Trans. Zool. Soc. vol. x. (1879) p. 441, pi. lxxiv. fig. 6. 
