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ERIK A : SON STENSIO 
sparse, straight and unramified striae, passing off transversally. These teeth resemble 
in their entire shape and development certain types of teeth belonging to Aster acanthus, 
e. g. those figured by Agassiz, vol. Ill, PI. 17, figs. 1—5. 
Nothing certain can be said as to the dentition of the palato-quadrate, but it 
does not seem improbable that its teeth were partly somewhat more robust than those 
of the mandible. There seem also to be certain reasons for believing that the shape 
of these teeth too may have been to a certain degree different from that of the 
mandibular teeth. 
The microscopic structure of the teeth. — In accordance with Jaekel’s 
observations in P. angustissimus (Agassiz) we also find in the species under discussion 
from Spitzbergen (PI. 3 , figs. 17, 18) that the crown is entirely composed of ortho-dentine, 
with the exception of the peripheral layer, while the root consists of trabecular-dentine 
(cf. Jaekel, 1889). 
According to their positions in the transversal rows the teeth have of course 
different ages and are therefore at different stages of development. Thus, for instance, 
we find that in the transversal row b of the mandible described above only the crown 
is developed in the youngest tooth, and the crown there seems to consist, apart from a 
distinct layer of enamel, of only a very thin layer of ortho-dentine formed in immediate 
connection with the enamel. The crown thus has a large pulp cavity, which is open 
basally. On the next tooth of the same row the layer of ortho-dentine in the crown is 
thicker and the root is developed, although it consists only of a rather thin peripheral 
layer of trabecular-dentine. Its interior thus has a large pulp cavity which communicates 
with the still present pulp cavity in the crown. In the succeeding teeth of the same 
row the pulp cavity decreases more and more through the growth of the layer of 
ortho-dentine in the crown and the trabecular-dentine of the root, but still it does not 
disappear entirely in the oldest teeth (p, PL 3 , fig. 17). In these it is found as a narrow 
longitudinal cavity between root and crown. 
Jaekel (1889, p. 329) also described and figured a longitudinal cavity of this sort 
in P. angustissimus, but he definitely denies that it has the nature of pulp. Everything 
clearly indicates, however, that even in this species we are concerned with a last 
remnant of the pulp cavity and not, as Jaekel maintained, with Haversian canals or 
cavities arisen from a certain expansion of one or more of these that have met together. 
Dermal denticles. 
Dermal denticles occur in rather great numbers among the teeth and the carti¬ 
laginous parts, but they are poorly preserved. They are all relatively small, the length 
and height amounting to about i - 5 mm and 075—1 mm respectively. In shape they 
resemble nearest those described by me in Hybodus and Acrodus, but they differ from 
these by having the ridges on their lateral extended as rather long, slender processes 
backwards. The number of these processes cannot be stated with certainty, but it is 
probably not more than three or four as a rule. 
