26 
ERIK A : SON STENSIO 
from the anterior to the posterior edge, at the distal end in the same direction only 
about 18 mm. The appearance of the cross section at both ends is seen in text fig. io A, B. 
The posterior edge (PI. i, fig. 18 a) has been equipped with denticles, but only the 
basis of a few of these are preserved. The lateral surfaces are without the usual ridges 
and all coarse ornament, but this is probably a secondary condition and due to 
weathering. The fine longitudinal striation that is present and whose character is clearly 
shown by PL i, figs. 18 a, b, is presumably found in all the fin-spines that, like this one. 
are built up in their external parts of trabecular-dentine. 
This outer layer of trabecular-dentine is in the present case 
thick and strong (o, text fig. io A). Inside it there follows a layer 
of ortho-dentine ( d , text fig. 10A), which becomes thicker towards 
the distal end of the spine in the way Agassiz has described 
(1843, vol. Ill, p. 215). The tubules of this ortho-dentine are somewhat 
irregularly and strongly ramified. The pulp cavity is much reduced 
in the part present ( p , text figs. 10 A, B) and really consists only 
of a fine vertical canal. The space between this and the ortho¬ 
dentine is filled up with a layer of trabecular-dentine (cq, text 
fig. 10 A) of the same type as in the outermost layer. As far as 
one can judge, this inner layer of trabecular-dentine has, however, 
been thinner towards the point of the spine, while, as has been 
mentioned, the ortho-dentine is thicker there. 
According to Agassiz (1843, pp. 212, 215, Tab. A, figs. 8, 9) the 
Hybodus-Yike fin-spines lack the central layer of trabecular-dentine 
that was found in the fin-spine P. 4 2 described above. As our 
knowledge of the structure of fin-spines in general is very in complete, it is not yet 
possible to decide whether this layer is of any importance from a systematic point of view. 
The fin-spine P. 102 is the one mentioned by Woodward 1912 (p. 291). It is re¬ 
presented by three fragments, one of which shows the shape of the pulp cavity at its 
proximal part. 
The spine has'been rather large and its maximum width (from the anterior to the 
posterior edge) is, as Woodward states, about 3 o mm. The sculpture is of the usual type. 
Its appearance is seen from PL 1, fig. 15. 
With regard to its finer structure it is noteworthy that no traces of any central 
layer of trabecular-dentine are found. It is also worthy of mention that the layer of 
ortho-dentine is absent round the proximal part of the pulp cavity, while it is present 
further distally and becomes rather thick towards the point. — I have also been able 
to ascertain that at least the fin-spine P. 282 described above has no layer of ortho¬ 
dentine at its proximal end. 
The fin-spine P. 883 is very incomplete (Pl. 1, fig. 16) and no reliable measurements 
can be given. The posterior edge has, as usual, a double row of thorn-like denticles. 
The ridges are finer than on the preceding fin-spine. The pulp cavity in the preserved 
part is rather large and is surrounded by a distinct and fairly thick layer of ortho-dentine. 
Test fig. IO. 
Cross section through 
fin-spine P. 4 . 2 . 1/1. 
A. Cross section through 
the proximal end of the 
preserved part. B. Cross 
section through the distal end 
of the same part, d, dentine 
of normal character (ortho¬ 
dentine); o, trabecular-den¬ 
tine, outer layer; o„ trabe¬ 
cular-dentine, inner layer; 
P, pulp cavity. 
