454 A. S, Woodward—British Jurassic Fishes. ; 
not prove to have been possessed of a formidable dentition. It is 
satisfactory to know that there is good reason to hope for the dis- 
covery of much more of the skeleton of the individual discnssed 
above, as soon as the bed where it occurs is worked again; and Mr. 
Leeds is fortunately acquainted with the precise stratum where the 
specimen occurs. 
The characters of the gill-rakers, branchiostegal rays, and pectoral 
fin-rays, taken together, justify the definite separation of the fish in 
question from all known generic types; and it is proposed to apply 
to it the name of Leedsichthys in honour of its discoverer. The 
Peterborough species may be provisionally termed Leedsichthys 
problematicus, and it is probably the most gigantic Jurassic fish 
hitherto described. 
A group of the characteristic gill-rakers, of equally large size, 
has also been obtained from the Oxford Clay of Vaches Noires (Brit. 
Mus. No. 32,581), thus indicating the occurrence of the genus in the 
Upper Jurassic of the North of France. 
6. Mesodon. 
The genus Pycnodus, as now defined, is restricted to the Eocene 
formations, and all the British Mesozoic fossils originally described 
under that name are to be distributed among the more precisely 
defined genera determined on the continent. This is a difficult task, 
so far as the Jurassic species are concerned, for little more than 
detached examples of jaws and teeth are known, and there is appar- 
ently considerable variation in these parts. The so-called Pycnodus 
pagoda, Blake,' fiom the Portlandian, is evidently a vomer of Micro- 
don; but nearly all the other described British Jurassic “ species ” 
of Pycnodus pertain to Mesodon. Fricke, v. Zittel, and others, have 
already pointed out that to this genus may be referred the Agassizian 
species P. Buckland, P. ovalis, and P. rugulosus, and to the synonymy 
of the first we would add P. didymus, Ag., P. obtusus, Ag., and 
Gyrodus perlatus, Ag. The latter name is given to some detached 
scales from the Stonesfield Slate, ornamented by tubercles instead of 
rugosities or pits, thus being truly referable to Mesodon, and agreeing 
sufficiently in size with the associated jaws of M. Bucklandi to be 
provisionally ascribed to that form. To M. rugulosus we would also 
assign the undescribed Pycnodus parvus, Ag., of which a specimen 
marked as “type” is in the Egerton Collection. Some so-called 
species of Gyrodus, e.g. G. trigonus, Ag., are also most probably 
referable to the same genus; and the Liassic Pycnodus liassicus, 
Kgert., was long ago placed in Mesodon by Heckel. 
7. Thrissops. 
Since the researches of Agassiz, Miinster, Wagner, and Thiolliére, 
so many Jurassic examples of the genus Thrissops have been 
acquired by various Museums. that it would be interesting to study 
the characters of the specific types already determined in the light 
of the new material before making any further additions to the 
1 J. F. Blake, “On the Portland Rocks of England,’’ Quart. Journ. Geol. Soc., 
vol, xxxvi. (1880), p. 228, pl. x. fig. 10. 
