TRIASSIC FISHES FROM SPITZBERGEN 
189 
The exit of the roots of the spinal nerve in each segment shows that in the ele¬ 
ments Id we are concerned with interdorsals (cranial neural elements) and in the ele¬ 
ments Bd with basidorsals (caudal neural elements) in the signification given to these 
terms by Schauinsland (1905). 
If we now turn to the haemal arches, we shall find these represented partly by 
robust four-sided skeletal elements ( Iv , text fig. 67 A.; PI. 21, fig. 4) and partly by long 
ventro-caudally pointing elements, Bv. To each skeletal element Iv corresponds an ele¬ 
ment Bv. In the most 
anterior part of the 
caudal region each 
element Bv is in 
connection with the 
ventralmargin of each 
element Iv. Towards 
the back, however, 
this state of affairs 
is altered, the con¬ 
nection being dis¬ 
placed first to the 
postero-ventral cor¬ 
ner and then still 
farther backwards to 
the posterior margin 
of the elements Iv. 
Each elements iV 
has a dorsal part 
that extends upwards 
along the chorda (pc, 
text fig. 67 A; PL 21, 
fig. 4), and a ventral 
part situated entirely 
under this. The dorsal 
part, consisting of a 
spongy bone sub¬ 
stance, is best deve¬ 
loped in the anterior 
part of the caudal region, where it has a characteristic four-sided shape, and a height 
corresponding to about half the diameter of the chorda. As one approaches the caudal 
fin it becomes gradually lower with each segment and consequently assumes a somewhat 
rectangular shape. The ventral part of each element Iv is also highest in the anterior 
end of the caudal region, and its shape is rectangular there, gradually becoming trapezoidal 
in the segments following backwards, because the anterior margin decreases in height. 
■Contrary to the dorsal part the ventral one consists of a rather compact bone substance 
like the elements Bv and the elements of the neural arches. Both parts are incompletely 
