PECTORAL SKELETON OF TELEOSTEANS. 373 
It is present during development in the cod and herring, but 
T have failed to findit in either Siphonostomaor Anguilla. 
Both the latter fish, however, are of a very special type, and 
are characterised by the great reduction of cartilage in all 
parts of the skeleton. Ina larva of Amia 19 mm. long there 
is no sign of it. 
- To Wiedersheim’s objection to Swirski’s interpretation it 
may be added that whereas in those forms which, according 
to Gegenbaur, have a well-developed “under process” it is 
closely associated with the cleithrum, the postcoracoid process 
is never, at any stage, related to that bone. 
- Owing to the researches of Humphrey, Thacher, and Regan, 
no doubt remains concerning the similar nature of the median 
and paired fins. ‘This is perhaps best shown by the pelvic 
finof Psephurus (Regan). Inthis fish there is very little to 
ehoose in the anatomical details between the pelvic and the 
anal fins. In both each ray of the fin consists of three 
segments, which—using Bridge’s terminology—may be called 
proximal (axonost), mesial (baseost), and distal (marginal) 
radials. In both there is a tendency towards the fusion of 
the anterior proximal radials. ‘This is carried further in the 
pelvic than inthe anal fin. The plate formed by the fusion of 
these elements is the very primitive pelvic girdle. The 
essential community of structure between the pectoral fins 
and the median is not so obvious; but, bearing in mind the 
researches of paleontologists, e.g. on Cladoselache and 
Cladodus, we have no difficulty in seeing it between the 
pectoral and pelvic fins of Psephurus. The distal and mesial 
elements are still present in the fin, and, as pointed out by 
Regan, the metapterygium and girdle represent the proximal 
elements, though in a much more specialised condition than 
in the pelvic fin. The metapterygium, therefore, has the 
same origin as the girdle, and is fundamentally the posterior 
continuation of it. The postcoracoid process has precisely 
the same relationships (e.g. fig. 9), and may be regarded as 
the homologue of the metapterygium. 
Further facts may be given in support of this conclusion, 
