Raymond C. Osburn 189 
for figure of this fin), in which the pelvic fin skeleton is in a condition 
similar to the anal of Chlamydoselachus at present). Regan’s important 
observations (04) on Psephurus are corroborative, as are also the earlier 
researches of Thacher, 77, 78; Mivart, 79, and Wiedersheim, g2. 
An intermediate condition between the pelvis of Chlamydoselachus and 
that of ordinary sharks (such as Scylhum, Spinax, Mustelus, etc.) is 
seen in the other Notidani (Hexanchus, Heptanchus). 
The pectoral girdle is of one type with very httle variation through 
the whole group of sharks. In Spinax and Cestracion, and probably in 
all sharks, it consists, at the time when it first becomes evident in the 
mesenchyme, of a short bar lying next to the ectoderm. Its upper end 
is continuous with the primary basal of the fin and there is at this time 
no dorsal (scapular) portion. In this condition the pectoral girdle cor- 
responds very closely to the pelvic girdle at the same stage of develop- 
ment. With later growth the dorsal part appears and the ventral end 
extends downward toward the mid-ventral line. The similarity of the 
two girdles is thus lost by the pectoral passing into a more specialized 
condition while the pelvic remains in a more primitive state. 
Professor Braus has recently (04a) tried to.show that the pelvic girdle 
is a degenerate structure in the adult, through the loss of a dorsal pro- 
cess which he homologizes with the scapular portion of the pectoral 
girdle, and therefore, of course, with the gill-arch. While we cannot 
doubt the accuracy of Professor Braus’s observation as to the occurrence 
of such a dorsal process in Spinax, we cannot agree to the interpretation 
he places upon it. The writer has elsewhere, (o6b) stated his objections 
to this view, which are, in brief: That the prominence in question is 
posterior to the nerve foramen, while the “scapula” is anterior in all 
sharks; that the lowest recent sharks known, the Notidanide, have no 
indication of such a prominence either in the adult or the young embryo; 
that the most ancient fossil shark of which the pelvic fin skeleton is 
known (Cladoselache) gives no indication of such a dorsal prominence. 
Whatever the aforementioned process in Spinax may represent, it cer- 
tainly cannot be the vestigial homolog of the “ scapula ” unless the nerve 
foramen has shifted from the posterior to the anterior side of the process, 
and even then we should have the paradox of the most degenerate condi- 
tion known in the shark mb occurring at once in the oldest fossil, and 
in the most primitive recent, sharks. 
Concerning the discovery by Ruge, 02, and Braus, o4a,—and the writer 
has verified this discovery—that in Spinax the first part of the pro- 
cartilaginous anlage of the pectoral fin skeleton to appear is the girdle, it 
