470 J. FRANK DANIEL 
The question has been raised: Do the diplospondylous verte- 
brae return to the monospondylous type as the tip of the tail is ~ 
approached. ‘This is stated to occur in Acanthias by Ridewood 
99), who says, ‘“The change from diplospondylous to the mono- 
spondylous condition occurs at about the twenty-fourth centrum 
from the end.” In Heterodontus francisci such is not the case 
(fig. 17). It is difficult, as Ridewood says, to delimit a myomere 
in the posterior region because of the thinness of the muscle, 
yet in an injected specimen the segmental arteries arising from 
the caudal aorta are clearly definitive of boundaries, only two 
to amyomere, one in front of it, the other bounding it posteriorly. 
Since two vertebrae occur between each two arteries, at least 
as far back as the ninety-eighth segment, the diplospondylous 
nature is retained. Some of the segments posterior to this 
retain their regularity although in these the arteries themselves 
are not sufficiently regular to be definitive. 
II. THE APPENDICULAR SKELETON 
The part of the skeleton known as appendicular consists of 
pectoral and pelvic girdles and the frame-work for the fins 
attached thereto. 
1. The skeleton of the fin girdles 
a. The pectoral girdle in Heterodontus (fig. 8) is a strong 
arch, open dorsally, to which the frame-work of the pectoral 
fin is attached. It is composed of a right and a left cartilaginous 
part solidly fused in the middle line below. The part of the 
girdle which extends dorsal to the attachment of the fin is 
the scapular portion (sc.). That part which meets a similar part 
from the opposite side below is the coracoid portion (co.). At 
the middle of the postero-lateral portion of each half of the girdle 
is a projection or articular surface (a.pt.) which fits into the 
fossa of the pectoral fin skeleton. In front of this articular 
surface is a strong antero-ventral projection (a.pr.), like that in 
Crossorhinus (see Haswell ’84), to which is attached strong muscu- 
lature. A foramen through which nerves and blood vessels pass 
