668 ICHTHYOLOGY [GEOGRAPHICAL Thanks to the researches of Agassiz, and especially of Sir P. Egerton, the ichthyological fauna of the Lias is perhaps the best known of the Mesozoic era, one hundred and fifty-two species having been described. Of the various localities, Lyine Regis has yielded more than any other, nearly all the Liassic genera being represented there by no fewer than seventy-nine species. The Hybodonts and Cestracionts continue in their fullest development. Holocephales (Ischyodus), true sharks (PalceoscyUium), rays (Squaloraia, Arthropterus), and stur geons (Chondrosteus) make their first appearance ; but they ars sufficiently distinct from living types to be classed in separate genera, or even families. The Ganoids, especi ally Lepidosteoids, predominate over all the other fishes : Lepidotus, Semionotus, Pholidophorus, Pachycortmis, Eugna- thus, Tetragonolepis, are represented by numerous species ; other remarkable genera are Aspidorhynchus, Belonostomus, Saurostomus, Sauropsis, Thrissonotus, Conodus, Ptycholepis, JSndactis, Oentrolepis, Legnonotus, Oxygnathus, Heterolepi- dotits, Isocolum, Osteorhachis, Mesodon. These genera afford evidence of a great change since the preceding period, the majority not being represented in older strata, whilst, on the other hand, many are continued into the succeeding Oolitic formations. The homocercal termination of the vertebral column begins to supersede the heterocercal, and many of the genera have well-ossified and distinctly segmented spinal columns. The cycloid form of scales also becomes more common, one genus (Leptolepis) being, with regard to the preserved hard portions of its organization, so similar to the Teleosteous type that some palaeontologists refer it (with much reason) to that subclass. As already mentioned, the Oolitic formations show a great similarity of their fish fauna to that of the Lias; but still more apparent is its approach to the existing fauna. Teeth have been found which cannot even generically be distinguished from Notidanus. The rays are represented by genera like Spathobatis, Belemnobatis, Tkaumas ; the Holocephaliare more numerous than in the Lias (Isc/tyodus, Ganodus). The most common Ganoid genera are Caturus, Pi/cnodus, Pholidophorus, Lepidotus, Leptolepis, all of which had been more or less fully represented in the Lias. Cera- todus also is continued into it. The Cretaceous group gives clear evidence of the further advance towards the existing fauna. Teeth of sharks of existing genera, Carcharias (Corax), Scyllium, Notidanus, and Galeocerdo, are common in some of the marine strata, whilst Hybodonts and Cestracionts are represented by a small number of species only ; of the latter one new genus, Ptychodus, appears and disappears. A very characteristic Ganoid genus, Macropoma, comprises homocercal fishes with rounded ganoid scales sculptured externally and pierced by prominent mucous tubes. Calurus becomes extinct. Teeth and scales of Lepidotus (with Sphcerodus as subgenus), clearly a freshwater fish, are widely distri buted in the Wealden, and finally disappear in the Chalk ; its body was covered with large rhomboidal ganoid scales. Gyrodus and Aspidorhynchus occur in the beds of Voirons, Coelodas and Amiopsis (allied to Amia) in those of Comen, in Istria. But the Palceichthyas are now in the minority ; undoubted Teleosteans have appeared for the first time on the stage of life in numerous genera, many of which are identical with still existing fishes. The most of these are Acanthopterygians, but Physostomes and Plectognaths are likewise well represented, most of them being marine. Of Acanthopterygian families the first to appear are the Berycidce, represented by several very distinct genera : Eeryx ; Pseudoberyx, with abdominal ventral fins; Berycop- sis, with cycloid scales; Homonotus, Stcnostoma, Spheno- cephalus, Acanm, Ifoplopteryx, Platycornus, with granular ecales ; Podocys, with a dorsal extending to the neck ; Acrogaster, Macrdepis.Rhacolepis, from the Chalk of Brazil. The position of Pycnosterynx is uncertain ; it approaches certain Pharyngognaths. True Percidce are absent, whilst the Carangidce, Sphyrcenid<x, Cataphracti, Gobiidce, Coif tidoe, and Sparidce are represented by one or more genera. Somewhat less diversified are the Physostomes, which belong principally to the Clupeidce and l)ercetidce, most of the genera being extinct; Chipea is abundant in some localities. Scopelidce (Hemisaiirida and Saiiroceplicdus) occur in the Chalk of Comen in Istria, and of Maestricht. Of all Cre taceous deposits none surpass those of the Lebanon for the number of genera, species, and individuals ; the forma are exclusively marine, and the remains in the most perfect condition. In the Tertiary epoch the Teleosteans have almost entirely replaced the Ganoids ; a few species only of the latter make their appearance, and they belong to existing genera (Lepi- dosteus, Acipenser). The Chondropterygians merge more and more into recent forms ; Holocephcdi continue, and still are better represented than in the present fauna. The Teleosteans show even in the Eocene a large proportion of existing genera, and the fauna of some localities of the Miocene (Oeningen) is almost wholly composed of them. Of the whole more than one-half have already been found to belong to existing genera, and there is no doubt that the number of distinct genera now seemingly extinct will be lessened as the fossils come to be examined with a better knowledge of the living forms. The distribution of the fishes differed widely from that of our period, many of our tropical genera occurring in localities which are now in cluded within our temperate zone, and being mixed with others that nowadays are restricted to a colder climate, a combination which continues throughout the Pliocene. A few families of fishes, like the freshwater /Salmonidce, seem to have put in their appearance in Post-Pliocene times ; not much attention, however, has been paid to fish- remains of these deposits ; and such as have been incident ally examined furnish evidence of the fact that the distri bution of fishes has not undergone any further essential change down to the present period. GEOGRAPHICAL DISTRIBUTION. In an account of the geographical distribution of fishes the freshwater forms are to be kept separate from the marine. When we attempt, however, to draw a line between these two kinds of fishes, we meet with a great number of species and of facts which would seem to render that distinction very vague. There are not only species which can gradu ally accommodate themselves to a sojourn in either salt or fresh water, but there are also some that seem to be quite indifferent to a rapid change from the one into the other ; so that individuals of one and the same species (Gastrosteus, Gobius, Blennius, Osments, Retropinna, Clupea, Syngna- thus, &c.) may be found some distance out at sea, whilst others live in rivers far beyond the influence of the tide, or even in inland fresh waters without outlet to the sea. The majority of these fishes belong to forms of brackish- water fauna ; and, as they are not an insignificant portion of the fauna of almost every coast, we shall have to treat of them in a separate chapter. Almost every large river offers instances of truly marine fishes ascending for hundreds of miles of their course, and not periodically, or from any apparent physiological necessity, but sporadically throughout the year. This is evidently the commencement of a change in a fish s habits; and, indeed, not a few of such fishes have actually taken up their permanent residence in fresh waters, as species of Ambassis, Apogon, Dules, Therapon, ScicMia, Blennius, Gobius, Atherina, Mugil, Myxus, Hemirhamphus, Clupea,
Anguilla, Tetrodon^ Trygon, all forms originally marine.
