The family of Berycidæ, equally interesting with regard to their distribution in time and in space, consists of temperate and tropical genera. The genus by which this family is represented in the southern temperate zone (Trachichthys) is much more nearly allied to the northern than to the tropical genera.
The true Cottina and Heterolepidina (forms with a bony stay of the præoperculum, which is generally armed) have not crossed the tropical zone; they are replaced by fishes extremely similar in general form, and having the same habits, but lacking that osteological peculiarity. Their southern analogues belong chiefly to the family Trachinidæ, and are types of genera peculiar to the Southern Hemisphere.
The Discoboli of the Northern Hemisphere have likewise not penetrated to the south, where they are represented by Gobiesocidæ. These two families replace each other in their distribution over the globe.
Nearly all the Pleuronectidæ (but they are not numerous) belong to distinct genera, some, however, being remarkably similar in general form to the northern Pleuronectes.
With Gadoids Myxinidæ reappear, one species being extremely similar to the European Myxine. Bdellostoma is a genus peculiar to the southern temperate zone.
As in the northern temperate zone, so in the southern, the number of individuals and the variety of forms is much less than between the tropics. This is especially apparent on comparing the numbers of species constituting a genus. In this zone genera composed of more than ten species are the exception, the majority having only from one to five.
The proportion of genera limited to this zone is rather high; they will be indicated under the several districts, which we distinguish on geographical rather than zoological grounds.
1. The Cape of Good Hope district.
The principal genera found in this district are the following (those limited to the entire zone being marked with a single (*) and those peculiar to this district with a double (**) asterisk):—
Chimæra, *Callorhynchus, Galeus, **Leptocarcharias, Scyllium, Acanthias, Rhinobatus, Torpedo, Narcine, Astrape, Raja.