ERIK A : SON STENSIO 
they are only rarely found there. Isolated finds of Hybodus species are also described 
from the Triassic of North-West America (Wemple, 1906, pp. 72—75; Jordan, 1907, 
pp. 98—99) and South Africa (Broom, 1909 a, p. 252), and I have myself shown the 
existence of a species in the Triassic of Spitzbergen (Stensio, 1918b, p.76). In the Jurassic 
the genus is represented by numerous species and in the lower Cretaceous several 
species still seem to have existed. With a few exceptions the Jurassic and Cretaceous 
species are all described from Europe. 
Almost all the better known Hybodus species are obtained from the Jurassic and 
the Cretaceous (Wealden). Among the Triassic ones H. africanus (Broom, 1909 a) is the 
only one that is somewhat completely preserved and that shows the general shape of 
the body with parts of the axial and fin skeleton. In all the Triassic species the dentition 
is so far known only from detached teeth, and there is consequently not a little un¬ 
certainty about this in several respects, a fact that has caused and still causes great 
difficulties, for the determination of the species. 
As we know, Agassiz had originally grouped together a number of rather different 
types of teeth from the Triassic into one common species, for which he chose the name 
of H. plicatilis. When later on, some remains of jaws of the Jurassic species H. reticulaius 
came into his hands, he was struck by the fact that the teeth on these remains appeared 
to be of a very homogeneous character. Under these circumstances he considered that 
it Wjas , impossible to retain the species name H. plicatilis in its earlier sense. He limited 
it to one of the types of teeth and suggested for the others the names H. mougeoti , 
H. polycyphus and H. angustus. In addition of these species he also described a number 
of others from the Triassic; of these H. longiconus, H. obliquus and H. cuspidatus ought 
to be mentioned in this connection. 
At the present time there is certainly agreement in considering Agassiz’s division 
as being carried too far in this case and that some of the names proposed by him 
ought to be regarded as synonyms, but with regard to the actual limits of species 
there is still a great difference of opinion. Schmid has suggested (1861, pp. 18—20), if I 
understand his exposition correctly, a grouping of all the Triassic species, described by 
Agassiz, under the name of H. plicatilis and this view is maintained later on by Erech 
(1903, pp. 10 — 11). Woodward, on the other hand, proceeds more cautiously. Thus in 
his «Catalogue» of 1889 we find that of Agassiz’s species he has adopted H. plicatilis 
and H. longiconus under the common name of H. plicatilis and in the same way H. mougeoti 
and H. obliquus under the name of H. mougeoti, while the other species are provisionally 
retained as defined by Agassiz 1843. At the same time Woodward emphasized the im¬ 
possibility of arriving at any certain view as to the precise character of the dentition 
merely, with the help of detached teeth. Jae^el (1889) incorporated H. plicatilis 
in its. restricted signification with H. longiconus. He also established two 
new species, H. multiconus and H. multiplicatus, but remarks that there was really 
no objection to at fusion of these with H. longiconus-, he unites ' H. polycyphus and 
H. mougeoti into one species, on which he base's the new genus Polyacrodus. In 
a later work (1898) he has definitely decided to unite the three species H. longi¬ 
conus, H. multiconus and H. multiplicatus. Another view is held by Sauvage (i 883 , 
pp. 495—496) and still other views could be, mentioned, but what has already been 
