Raymond C. Osburn 187 
comes into contact with it. In short, the first anlage of the pectoral 
girdle is wholly situated within the domain of somatic muscles and the 
trapezial connection is accomplished later by the dorsal growth of the 
girdle and the postero-ventral growth of the muscle. If the time ele- 
ment is at all trustworthy here (and we must believe so until it is proved 
otherwise} the above facts can only be interpreted to mean that the 
ventral portion of the girdle is more primitive than the dorsal, and, if 
this is true as the development indicates, then at one time the pectoral 
girdle consisted only of a ventral portion and was in somewhat the same 
condition as the pelvic girdle in most fishes to-day. 
The discovery of this early condition of the pectoral girdle, I con- 
clude, shows its complete distinctness from a gill-arch and places it in 
the same category with the pelvic fin. The gill-arch theorists have gener- 
ally considered the pelvic fin as simplified from a condition still retained 
in the pectoral, or, in other words, as the more modified of the two,— 
a deduction necessarily following their assumption as to its mode of 
origin. But the facts indicate clearly that the pelvic fin represents the 
more primitive type, and that the pectoral fin passes through a similar 
stage of development and then progresses beyond this to the condition 
seen in all recent sharks. 
The connecting links between the pelvic and unpaired fin skeletons 
have been so clearly presented (Thacher, 77, 78; Mivart, 79; Wieders- 
heim, 92; Regan, 04) as to need no particular comment here. I wish 
to bring up the case of Chlamydoselachus in this connection, however, 
because it has also a bearing on another point. The adherents of the 
Gegenbaur theory have tried to find some dorsal projection of the pelvic 
girdle to homologize with the scapular portion of the pectoral girdle, but 
the “ pars iliaca” of Davidoff, 79, 80, cannot be considered, and the 
“ nrocessus iliacus”’ of Braus, 04a, must also be thrown out of compari- 
son since it does not bear the same relation to the nerve foramen as the 
scapula does in the shoulder girdle. In Chlamydoselachus the pelvic 
girdle is a broad flat plate (see Fig. 20 from a camera drawing of a 
Van Wijhe preparation of a 225 mm. embryo) which serves also as a 
basale for about half of the rays of the fin. It is pierced by eight spinal 
nerves instead of a single collector. It has not the shghtest indication, 
even in a 120 mm. stage, of any dorsal prominence whatever. Such a 
condition is impossible of explanation under the gill-arch theory, for the 
pelvic of Chlamydoselachus cannot by the greatest stretch of the imagi- 
nation be made to homologize with a gill. In fact it resembles nothing 
so much as it does the flat, plate-like basalia of certain unpaired fins. 
14 
