DESCRIPTION OF THE FISH FAUNA. 
Subclass Rlasmobranchii. 
Family Cestracionidae. 
Genus Hybodus Agassiz. 
Synonyms: See Woodward, A. S., The Wealden and Purbeck Fishes, etc. Palaeontogr. Soc., Vol. 69, p. 3 . 
Under the generic name Hybodus Agassiz grouped together (xSSy, vol. 3, p. 41, 
p. 180) a number of species which he had established on the basis of a material consisting 
mainly of detached teeth and fin-spines. As the state of preservation only allowed him 
in exceptional cases to decide with certainty which teeth and fin-spines belonged to the 
same species, he described them separately under different names. 
The knowledge of the genus Hybodus increased rather slowly during the four 
decades after Agassiz’s description. The most important works during this period are 
those of Egerton (1845, pp. 197—199) and Day (1864, pp. 57—65). It was not until the 
beginning of the eighties that the knowledge of this genus began to increase more 
rapidly. 1889 was a particularly important year in this respect, as Woodward then 
published the first volume of his large «Catalogue of Fossil Fishes in the British Museum», 
in which both the genus Hybodus and the fossil sharks in general were thoroughly treated 
on the basis of an abundant material. In the same year there appeared Jaekel’s valuable 
work on the Selachii in the upper Muschel-kalk of Lorraine (1889, pp. 294—3og), in which 
Hybodus forms were also dealt with in detail. 
Among the numerous works that further have followed during the last thirty 
years I may mention those of Jaekel (1890; 1898; 1906 a), Fra as (1889), Brown (1900), 
Koken (1907), Broom (1909a), Vidal (1915) and Woodward (1915; 1917). 
As has been pointed out by Day (1864) and Woodward (1889 a, pp. 267, 279, 3oo), 
Hybodus is closely related to Acrodus. Certain of the teeth may be developed fairly 
similarly in both genera, and in the case of detached cephalic spines and fin-spines it 
is impossible to decide to which of the two genera they belong. 
The earliest representatives of the genus Hybodus appeared in the Triassic 1 ). They 
occur frequently in the continental Triassic of Germany, France and England. They 
have been shown to exist in the alpine Triassic deposits (de Alessandri, 1910, pp. 3i—34; 
Zugmeyer, 1875, pp. 79 — 80; Bittner, 1898, pp. 3z3—3z6; Wilckens, 1909, p. 188) 2 ) though 
J ) A tooth from the Kupferschiefer of Germany is described by Geinitz (1861, p. 27, pi. IV, fig. 4) as 
belonging to Hybodus, but his determination may be doubted. As far as one can judge from his figure the characters 
of the tooth seem to point to a relation with Polyacrodus. 
2 ) The species described by Bittner as Hybodus austriacus cannot really be a Hybodus. It seems also certain 
that it cannot belong to Polyacrodus, as Wilckens (4909) has supposed. Probably it may represent a new genus. 
Stensio, Triassic Fishes from Spitzbergen. I 
