ERIK A : SON STENSIO 
bitumen) found in the place occupied by the chorda may possibly constitute remains of 
its sheath. 
The neural and haemal arches are preponderatingly ossified and are situated fairly 
well in situ both relative to each other and to the space occupied by the chorda (text 
fig. 67 A; PI. 21, fig. 4). 
The neural arches consist in each segment of two paired elements. One of these 
elements (Id) is trapezoidal in shape with the shortest margin directed dorsally and the 
longest, which rests against the chorda, ventrally, while the anterior and posterior 
margins are so situed that they converge upwards. In the segments belonging to the 
most anterior part of the caudal region the height of the elements Id is only slightly 
smaller than the length, and in these segments the antero-dorsal corner of each of these 
elements has also a rather deep sinus. Posteriorly, towards the middle and posterior 
parts of the caudal region the height of the elements Id decreases gradually with each 
segment, and the above-mentioned sinus becomes larger and more shallow, finally 
occupying nearly the whole of the dorsal margin. 
The other element ( Bd , text fig. 67, PL 21, fig. 4) in each neural arch is considerably 
more strongly ossified than the one just described and, unlike it, very much drawn out 
in height. It is no placed vertically but is inclined backwards and this inclination in¬ 
creases the closer one approaches to the caudal fin. With regard to function we may 
conveniently distinguish in this element a short, widened proximal part, which forms 
part of the bouudary of the neural canal, and a distal part, which forms a long 
neural spine. Both these parts pass into each other without any distinct boundary, 
and the element as a whole has a shape that most closely resembles a rifle with the 
butt towards the chorda. The proximal part extends in between two adjacent elements 
Id and has presumably reached down to the chorda, though it can have come into 
direct contact with this only over a rather short stretch. Its relations to the elements 
Id are shown in text fig. 67 and in PL 21, fig. 4. 
In the anterior part of the caudal region we find that the elements Bd have on 
their caudal (ventro-caudal) margin a weak, though distinct, sinus, situated opposite the 
sinus on the anterodorsal corner of each element Id following immediately behind. On 
the elements Bd that fall within the middle and posterior part of the caudal region the 
sinus gradually becomes indistinct and finally can no longer be distinguished. In the 
abdominal region, on the other hand (P. 157; text fig. 67 B; Pl. 22, fig. 2), the sinus is 
very strong and is bounded dorso-caudally by a narrow downward-pointing process (pi). 
On the anterior (antero-dorsal) margin too the proximal part of the element Bd has 
a sinus, bounded posteriorly and at the top by a small process (p). This sinus is found 
clearly developed both in the abdominal an the whole caudal region. 
The sinus in the antero-dorsal corner of each element Id together with the sinus 
in the caudal margin of each element Bd situated in front, has obviously formed a 
foramen, which has apparently been pierced by the ventral spinal nerve root. The dorsal 
root may have been transmitted dorso-caudally of the ventral one presumably close to 
the anterior margin of each element Bd. It is not impossible that it may then have had 
some connection with the sinus on the anterior (antero-dorsal) margin of the proximal 
part on the element Bd. 
