28 
ERIK A : SON STENSIO 
versally are somewhat coarse and sparse and do not reach the basal margin of the 
crown. Other striae, also transversal, in the neighbourhood of this margin are finer and 
situated closer together. There is usually a stronger stria crista running in a basal 
direction on the lateral and medial sides of the cones, giving these a more or less distinct 
pyramidal shape. — The dental crown is almost entirely formed of ortho-dentine, con¬ 
trary to what is the case in Hybodus and Acrodus. 
The microscopic structure of the crown is considered by Jaekel as the most im¬ 
portant characteristic of the genus, but he adds at the same time it may easily be thought 
that this structure has arisen from the one found in Acrodus. «Man hat eben nur anzu- 
nehmen», he writes, «daS die auch bei Acrodus vorhandene Dentinzone bei Polyacrodus 
sehr machtig entwickelt ist.» 
In a paper published in 1898 Jaekel somewhat modified his original definition of 
Polyacrodus. He now limits it (pp. 137—138), as he says, to the teeth, «bei denen sich ein 
zusammenhangender Langskiel fiber samtliche Zahnkegel zieht und die Faltelung der 
Seitenflachen von den Zahnspitzen ausgeht*. Eight years later he considers (1906 a, p. 158) 
that, on account of the nature of the teeth, he ought also to include in the genus 
Polyacrodus the species from the Lias described by Fraas as Hybodus hauffianus. 
As has already been pointed out in this work (p. 3 above) Koken found (1907) 
that H. hauffianus had the teeth developed as in the typical Hybodus species with regard 
to the microscopical structure and that H. hauffianus in all other respects as well could 
not be separated from the genus Hybodus. He further remarks that we must attribute 
a greater scope of variation not only to the outer shape of the Hybodus teeth but also 
to their microscopical structure, especially with regard to the degree of development of 
the dentine. In accordance with this he has considered Polyacrodus as a synonym of 
Hybodus in his treatise of the fishes in the second edition of Zittel’s «Grundzfige der 
Palaontologie» (1911). 
Finally Schlosser (1918, p. 59) has adopted the genus Polyacrodus, but restricted it 
to include teeth from the Triassic alone. 
Polyacrodus has hitherto only been known from Germany and the east of France, 
where it has been found especially in the Muschelkalk and the lower Keuper (cf. above 
p. 1, note 2). 
It is now very remarkable that in the Triassic of Spitzbergen two species occur 
which in their characters resemble both Hybodus, Acrodus and Palaeobates. As far as I 
can see they come within the bounds of Jaekel’s definition of the genus Polyacrodus and 
I thus consider it certain that this genus ought to be maintained. At the same time it 
seems to me also necessary to confine it to Triassic forms, as Schlosser did. 
The new material from Spitzbergen affords of course a somewhat extended know¬ 
ledge of Polyacrodus and I have therefore to add to Jaekel’s original definition the 
statement that the dentition was probably rather heterodont. With regard to the ornament 
of the teeth it ought to be expressly stated that this may be very feebly developed or 
be quite absent, and that it consists of striation mainly issuing from the longitudinal keel. 
In this connection it is worth nothing that one of the Polyacrodus species described 
below, P. angularis, resembles especially closely certain of the types of teeth that Quenstedt 
(1852, p.179), Eck (1865, p. 118) and Woodward (1889a, p.282) have grouped together 
