466 J. FRANK DANIEL 
nounced fashion. In both dorsals and ventrals the body tapers 
toward the free end. The fourth extra branchial (not added in 
fig. 14) is smaller and simpler in form both above and below, 
than the preceding. 
The four dorsal extra-branchial segments are united in such a 
way as to make the upper attachment in a continuous line above 
the gill pockets. Each of these cartilages curves around the tip 
of the branchial rays at the margin of a gill septum. The 
attachment of the lower or ventral extra branchials is less con- 
centrated. The first three of these are joined to the connective 
tissue ventrally at the base of their respective gill-pockets, but 
the fourth is shorter and tends to migrate upward so as to rise 
from the side of its ceratobranchial. 
2. The spinal column 
The spinal column in Heterodontus, although somewhat 
variable in the number of its segments, consists of about one 
hundred and ten clearly marked vertebrae. Of these the first 
thirty-one have ribs growing from their basiventrals (6.v.) or 
transverse processes; the five or six following these have their 
basiventrals bent downward, and the sixth or seventh (thirty- 
seventh or thirty-eighth of the column) usually has them meeting 
below to form the first hemal arch. Thirty-four similar arches 
follow back from the first haemal arch to the beginning of the 
ventral rays of the tail (on the seventy-first vertebra). There 
are next thirty-nine or forty caudal vertebrae, behind which in 
the adult is a mass but slightly differentiated, tapering gradually 
to a point (see also T. J. Parker ’87, p. 31, pl: 8, fig. 28). 
The first vertebra in the column (fig. 15) is obscured by an 
incomplete anterior segment having a short centrum and bearing 
’ enlarged transverse processes for articulation with the occipital 
condyles of the cranium. The second complete vertebra (vt.?, 
fig. 15), appearing behind the incomplete segment, may be de- 
scribed as provided with a strong centrum upon which rests a 
basidorsal (basal) piece (b.d.2), above and posterior to which is a 
Jarge interdorsal (intercalary) plate (7.d.2).. Both of these plates 
are perforated, the former giving passage to the ventral root 
