1066 
SCANDINAVIAN FISHES. 
the vent- and advance along each side of the belly". 
Yet this structure has undergone great development, 
retaining meanwhile the axial parts of the paired tins 
to such an extent that morphological comparison may 
have recourse to the Elasmobranchs in quest of starting- 
points for the interpretation of the arms and legs of 
the higher vertebrates. From embryology Balfour * 6 
and Wiedersheim" learnt — as Teacher'* and Mivart" 
had previously discovered by researches in comparative 
anatomy — that the primordial parts in the skeleton of 
the paired tins are the basal portions of the cartilagi- 
nous rays (primary radialia), which grow inwards to 
form the groundwork of the shoulder-girdle and the pel- 
vis, and in their outward collocation, radiating unilater- 
ally from a basal part, are split in a, transverse direction 
a still surviving Lung-fish The axial parts which have 
been retained in the paired tins of the living Elasmo- 
branchs have been named by Gegenbaur the anterior , 
middle , and posterior pteryyial parts {pro-, meso-, and 
metapterygium). In some forms, as in the Rays, the 
Notidanoids, and some other Sharks, all these parts 
share in the articulations of the pectoral tins, in others 
only two of them or even only one, as the proptery- 
gium and a part of the inetapterygium in ChimceYa 
(tig. 294, pp and mt t ), and the metapterygium in 
Scymnus. The last-mentioned peculiarity may also be 
observed in the ventral tins. In the Chondrosteans and 
Teleosts we have seen how these parts gradually dis- 
appear during the development of the normal piscine 
type. In the ventral tins of all male Elasmobranchs, 
Fig. 295. Left side of the pelvic girdle and left ventral fin of a Chimcera monstrosa, 2 /s nat. size. 
b , undivided basal cartilage of the ventral fin, answering to the propterygium and mesopterygium ; for , obturator foramen, at the side of 
which appear two fenestrae, closed with membrane, in the pelvic cartilage; pit, iliac part; pis. ischiadic part; pp, pubic part ; prp , prsepubic 
part, serrated cartilage or anterior copulatory organ of the male; ptpd, pterygopodium of the male, metapterygoid structure, consisting of: 
mti, inner metapterygoid cartilage; mf and mt ,, inner (proximal) and outer (distal) metapterygoid parts, the latter divided into three bran- 
ches, the outer superior ( se ), the inner superior (si) and the inner inferior (it). 
and multiplied within the tin itself. That the prim- 
ordial arrangement, however, here as in the cliphycercal 
caudal tin, was bilateral, with rays on each side of an 
axial part, appears from’ such forms as Fleur acanthus, 
a primitive Shark of the Permian fauna, or Ceratodus, 
on the other hand, the metapterygium is prolongated 
into free appendages (tig. 295, ptpd), which may serve 
as organs of motion, but are properly copulatory organs. 
The male Chiinseras are furnished with another ap- 
pendage, possessing the same function, a mobile, sickle- 
“ Cf. Smitt: Ur de hogre djurens utvecklingshistoria, pp. 240 and 241. 
6 Comparative Embryology , vol. II, pp. 492, cett. 
c Das Gliedmassenskelet der Wirbelthiere , Jena 1892. 
d Median and paired fins, Trans. Connect. Acad., vol. Ill (1877). 
e On the Fins of Elasmobranchii , Trans. Zool. Soc. Lond., vol. X. 
f Appending a reference to the structure of the paired fins in Ceratodus, I expressed in 1873 the above-cited opinion, which 
Wiedersheim (1. c., p. 13) calls “Die TuACHER-MivART-BALFOUR-HASWELL-DoHRN’sche Lehre” . 
