10 MONOGRAPH OF THE FRESH WATER III. 
marine or fluviatile, shows a very intimate relationship, both amongst themselves 
and the neighboring groups of Scorpznoids and Scizenoids. 
The typical genus of the marine division is that of Acanthocottus, numerous in 
species, and distributed over the arctic and temperate regions of both hemispheres. 
In the Pacific Ocean we have the genera Trachidermis and Podabrus, composed 
each of but a few species confined to the seas of China and Japan. The first is a 
diminutive of Acanthocottus, from which it differs in having a prickly skin and 
teeth on the palatine bones. The second is intermediate between Trachidermis 
and Acanthocottus, without forming, however, any direct passage from one to the 
other; it has the smooth skin of Acanthocottus and the palatine teeth of Trachi- 
dermis. As to the general form and physiognomy, one would consider Podabrus as 
the eccentric type of Acanthocottus: the two species which it embraces, differing 
greatly from each other in that respect. 
The genera Hemitripterus and Hemilepidotus exhibit a tendency towards Scor- 
penoids ; both are provided with teeth on the palatine bones, the skin of the former 
being somewhat prickly, and that of the second, partly covered with scales. 
The typical genus of the fresh water division is that of Cottus, numerous in species, 
distributed over the whole range of the temperate region of both hemispheres. On 
the north-western shores of America there is the genus Cottopsis, the amplification of 
Cottus, provided with a prickly skin and teeth on the palatines, like Trachidermis. 
Finally, the genus Triglopsis seems to recapitulate both Acanthocottus and 
Cottus, and to foreshadow the group. of Scizenoids. 
Thus eight genera, five marine, and three from the fresh water, constitute 
actually a natural group to which the name of Corroms is well appropriated. 
§ 2. GENEALOGY OF THE CorrToIDs. 
The history of the Cottoids prior to our epoch rests, as yet, only upon the dis- 
covery of a few fossils in Europe. The family appeared for the first time on the 
surface of the globe, sometime during the last period of the cretaceous epoch, where 
it was represented by two genera, Pterygocephalus and Callipteryx, precursors of the 
group of Triglide; their affinities leave no doubts on this point. But these two 
genera die out with the end of their epoch. 
To them succeeds the genus Cottus in the tertiary epoch, where it was the only 
one of the family, and, indeed, had but few known species. Two of these, Cottus 
brevis and C. papyraceus, Agass., come nearest to C. gobio and other fresh water 
species of the genus now living, and a third species, Cottus aries, Agass., is more 
intimately related to Cottus scorpioides and bubalis, precursor of the marine type of 
the genus, which we now call Acanthocottus. : 
It isnot improbable that Cottoids may have existed in North America before 
the present epoch. The study of the cretaceous and tertiary deposits situated 
within the limits of the zone inhabited by living species will furnish us information 
on this point. 
The geological researches on the Asiatic Continent, have not yet furnished us 
