410 The American Naturalist, [May, 
were specializations of the median folds of Branchiostoma, so the 
paired fins were specializations of the two lateral folds which 
are the homologues of the Wolffian ridges, in embryos of higher 
forms. Rods formed in both median and lateral folds. In the 
latter, these became at least twice segmented, the orad ones were 
prolonged inwards, more or less concresence took place, and the 
cartilages spreading, met below in the middle line, and later, ex- 
tending dorsad, completed the limb girdle. Gegenbaur, in the 
later edition of his “ Grundriss,” has modified his views respecting 
the nature of the primordial fin-rays, since he says : ” “ Supporting 
organs, formed from the integument, are connected with the parts 
thus formed by the vertebral column ( ior and inferior arches), 
and they are continued into the caudal fin. In the Selachii the 
fin-rays are formed by the so-called horny filaments, and in the 
Ganoidei and Teleostei by ossifications.” 
The same eminent anatomist, who has aided so greatly in es- 
tablishing the relationship between the various parts of the simple 
paired fins of the Dipnoi, and those of the air-breathing verte- 
brates (Stapedifera, Thacher), refuses to concede the homodyna- 
mism of the median and paired fins, but adheres in the later 
edition of the “ Grundriss ” to the theory propounded by him in 
1872,* namely: their derivation from the branchial arches, by the 
articulation of the simple lateral rays set upon those arches to 
the central ray, which gradually assumes increased length, and 
becomes an axis. In this way a fin like that of Ceratodus might 
be produced. 
The views of Mr. Thacher are, however, supported by the in- 
dependent but later observations of Balfour,* who found that in 
the embryo dogfish the lateral fins have precisely the same mode 
of origin as the dorsal median fin, arising “as special develop- ` 
ments of a continuous ridge on each side, precisely like the 
ridges of epiblast which form the rudiments of the unpaired 
fins.” 

23 Elements of oon Anatomy, by Carl Gegenbaur, 1877. (Translated by F. 
pey Bell, B.A., 1878), p 
2t Untersuchungen zar Vorgleichenden Anat. der Wirb., Hft. III., Kopfskelet der Se- 
lachier, p. 181r, Note, 
Balfour. biavlicates of Elasmobranch Fishes, 1878. 
