Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/685

667 DISTRIBUTION.] ICHTHYOLOGY 667 on a straight or curved base. Modifications of these ar rangements are very numerous, and many palaeontologists still entertain doubts whether the origin of these remains is not rather from annelids and mollusks than from fishes. The first undeniable evidence of a fish, or, indeed, of a vertebrate animal, occurs in the Upper Silurian rocks, in a bone-bed of the Downton Sandstone, near Ludlow. It consists of compressed, slightly curved, ribbed spines, of less than 2 inches in length (Onchus) ; small shagreen- scales (Thelodus) ; the fragment of a jaw-like bar with pluricuspid teeth (Plectrodus) ; the cephalic bucklers of what seems to be a species of Pteraspis ; and, finally, coprolitic bodies of phosphate and carbonate of lime, including recognizable remains of the mollusks and crinoids inhabit ing the same waters. But no vertebra or other part of the skeleton has been found. The spines and scales seem to have belonged to the same kind of fish, which probably was a Plagiostome. It is quite uncertain whether or not the jaw (if it be the jaw of a fish) belonged to the buckler- bearing Pteraspis, the position of which among Ganoids, with which it is generally associated, is open to doubt. No detached undoubted tooth of a Plagiostome or Ganoid scale has been discovered in the Ludlow deposits j but this much is certain, that those earliest remains in Palaeozoic rocks belonged to fishes closely allied to forms occurring in greater abundance in the succeeding formation, the Devonian, where they are associated with undoubted Paltfichthyes, Plagiostomes as well as Ganoids. These fish-remains of the Devonian or Old lied Sandstone can be determined with greater certainty. They consist of spines, the so-called &quot; ichthyodorulites,&quot; which show suffi ciently distinctive characters to be referred to several genera, one of them, Onchus, still surviving from the Silurian epoch. All these spines are believed to be those of Chondropterygians, to which order some pluricuspid teeth (Cladodus) from the Old Red Sandstone in the vicinity of St Petersburg have been likewise referred. The remains of the Ganoid fishes are in a much more perfect .state of preservation, so that it is even possible to obtain a tolerably certain idea of the general appearance and habits of some of them, especially of such as were provided with hard carapaces, solid scales, and ordinary or bony fin-rays. A certain proportion of them, as might have been expected, remind us, as regards their external form, of Teleosteous fishes rather than of any of the few still existing Ganoid types; but it is contrary to all analogy and to all palseontological evidence to suppose that those fishes were, in their internal structure, more nearly allied to Teleosteans than to Ganoids. If they were not true Ganoids, they may justly be supposed to have had the essential characters of Palceichthyes. Other forms even at that remote geological epoch exhibit so unmistak ably the characteristics of existing Ganoids that no one can entertain any doubt with regard to their place in the system. In none of these fishes is there any trace of verte bral segmentation. The Palceichthyes of the Old Red Sandstone, the systematic position of which is still obscure, are the Cephalaspidce from the Lower Old Red Sandstone of Great Britain and Eastern Canada; Pterichthys, Coccos- teus, Diuichthys, and Asterolepis (genera which have been combined in one group, Placodermce) ; and Acan- tkodes and allied genera, which combined numerous branchiostegals with Chondropterygian spines and a shagreen-like dermal covering. Among the other Devonian fishes (and they form the majority) two types may be recognized, both of which are unmistakably Ganoids. The first approaches the still living Polypterus, with which some of the genera like Diplopterus singularly agree in the form and armature of the head, the lepidosis of the body, the lobate pectoral fins, and the termination of the vertebral column. Other genera, as Iloloptychius, have cycloid scales ; many have two dorsal fins (Holoptychiut), and, instead of branchio stegals, jugular scutes ; others have one long dorsal con fluent with the caudal (Phaneropleuron). In the second type the principal characters of the Dipnoi are manifest ; and some of them for example, Dipterus, Palcedaphus, Holodus approach so closely the Dipnoi which still survive that the differences existing between them warrant a sepa ration into families only. Devonian fishes are frequently found under peculiar circumstances, enclosed in the so-called nodules. These bodies are elliptical flattened pebbles, which have resisted the action of water in consequence of their greater hardness, whilst the surrounding rock has been reduced to detritus by that agency. Their greater density is due to the dis persion in their substance of the fat of the animal which decomposed in them. Frequently, on cleaving one of these nodules with the stroke of the hammer, a fish is found embedded in the centre. At certain localities of the Devonian, fossil fishes are so abundant that the whole of the stratum is affected by the decomposing remains, emitting a peculiar smell when newly opened, and acquir ing a density and durability not possessed by strata with out fishes. The flagstones of Caithness are a remarkable instance of this. The fish-remains of the Carboniferous formation show a great similarity to those of the preceding. They occur throughout the series, but are very irregularly distributed, being extremely rare in some countries, whilst in others entire beds (the so-called bone-beds) are composed of ichthyolites. In the ironstones they frequently form the nuclei of nodules, as in the Devonian. Of Chondropterygians the spines of Onchus and others still occur, with the addition of teeth indicative of the existence of fishes allied to the Cestracion type (Cochliodiis t Psammodus), a type which henceforth plays an important part in the composition of the extinct marine fish faunae. Another extinct Selachian family, that of the Hybodonts, makes its appearance, but is known from the teeth only. Of the Ganoid fishes, the family Palceoniscidve (Traquair) is numerously represented ; others are Coslacanths (Ccel- amnt/ms, Rhizodus), and Saurodipterini (Megalichthys). None of these fishes have an ossified vertebral column, but in some (Megalichthys) the outer surface of the vertebrae is ossified into a ring ; the termination of their tail is heterocercal. The Carboniferous Uronemus and the Dev onian Phaneropleuron are probably generically the same ; and the Devonian Dipnoi are continued as, and well repre sented by, Ctenodus. The fishes of the Permian group are very similar to those of the Carboniferous. A type which in the latter was but very scantily represented, namely the Platysomidcs, is much developed. They were deep-bodied fish, covered with hard rhomboid scales possessing a strong anterior rib, and pro vided with a heterocercal caudal, long dorsil and anal, short non-lobate paired fins (when present), and branchiostegals. Palceoniscus appears in many species; the Sauridce are represented by Pygopterus and Acrolepis, and Cestracionts by Janassa and Strophodus. The passage from the Palaeozoic into the Mesozoic era is not indicated by any marked change so far as fishes are concerned. The more remarkable forms of the Trias are shark-like fishes represented by ichthyodorulites, like Nemacanthus, Liacanthus, and Ilybodiis ; and Cestracionts represented by species of Acrodus and Strophodus. Of the Ganoids, Coclacanthus, Amblypterus (Palceoniscida&amp;gt;), and Saurichthys persist from the Carboniferous epoch. Cera- todus appears for the first time (Muschel Kalk of Germany).