148 NATURAL HISTORY. 



Secondary strata : but in Tertiary deposits the Teleostean division is quite as well represented in the 

 geological formations as in existing seas. There is no evidence of any gradual succession of fishes 

 in the order of increased complexity of structure, as the deposits in which they occur approach nearer 

 to the present day. And there is no reason to suppose that the oldest fishes known were the 

 first that appeared upon the earth. The earliest fishes discovered are met with in the lower Ludlow 

 rocks, which form the upper part of the Silurian strata. The most ancient genus is Scaphaspis, a 

 small buckler-headed fish, which had the body covered with scales. Many allied genera are found in 

 the overlying Old Red Sandstone, in which fishes appear in extraordinary variety. Among the allies 

 of Scaphaspis are Pteraspis, Cephalaspis, Auchenaspis, and Didymaspis, some of which range down 

 to the Silurian rocks. Near to these fishes must be placed Coccosteus, Pterichthys, and the 

 immense American fossil of Devonian age, named Dinichthys. These fishes are thought to be 

 related to Ganoids and Sharks, but in external form they more closely approximated to Loricaria, 

 though the tail is heterocercal. They form a distinct group named Placodermi. Existing fishes, 

 however, with heterocercal tails, have the tail homocercal in an embryonic stage of development. 



The Ludlow bone-bed consists almost entirely of bones of fishes much triturated and matted 

 together, and very few species of fishes have been recognised in it, but among them is a Shark-like 

 fish spine referred to the cestraciont genus Onchus. The more striking of the Old Red Sandstone 

 fishes belong to the group which Professor Huxley names Crossopterygidse. This group of 

 ganoids comprises many fossil families in addition to the living Polypterus. Among them are 

 genera covered with rhomboidal scales, as in Polypterus, having two dorsal fins. The pectoral 

 fins have the rays arranged round a long central scaly portion, so as to form a fringe. Osteolepis, 

 Diplopterus, and Megalichthys are genera showing these chai-acters. This tribe is named Sauro- 

 dipterini. Another group, the Glyptodipterini, has sculptured scales, two dorsal fins, and the 

 pectoral fins greatly elongated. Some of the genera, such as Glyptopomus, Glyptolsemus, and 

 Gyroptychius, have the scales rhomboidal ; but other genera, such as Holoptychius, Glyptolepi^, 

 Platygnathus, have the scales cycloidal, and to this group probably belong Rhizodus, Dendrodus, 

 and other types. Another section formed for the genus Dipterus is termed Ctenodipterini. All 

 its fins are elongated, and look like lobes of the body. Its scales are cycloidal, and the teeth are 

 crossed by ridges. The genus Phaneropleuron has one long undivided dorsal fin and thin cycloidal 

 scales : its teeth are conical. The Coalacanthini is another remarkable group of these fishes, which 

 have the air-bladder large and ossified. It includes such forms as the Ccelacanthus and Macropoma, 

 which latter ranges through the Kinimeridge Clay to the Chalk. A section of the Ganoid order, 

 represented at the present day by Lepidosteus, appears in the Secondary strata to have attained an 

 immense development ; but while the living Lepidosteus has the maxillary bone divided into several 

 pieces, the fossil genera of the Lias and other Secondary formations have the maxillary bone in one piece. 

 Among the better-known fossil genera are such types as Lepidotus, Dapedius, Tetragonolepis, 

 Eugnathus, Pachycormus, and Aspidorhynchus. The fishes allied to Pycnodus have the jaws 

 covered with rows of flat-crowned teeth, adapted for crushing, with sharp incisor teeth in front, 

 Pycnodus ranges through the Secondary rocks up to the Tertiary, a species being found in the London 

 Clay of the Isle of Sheppey. Closely allied to these forms are genera from the Primary rocks, such 

 as Platysomus, in which, as in the Palseoniscidae, the vertebral axis is notochordal, the caudal fin is 

 heterocercal, and the scales have the ganoid character. The fishes resembling Platysomus have a 

 short and deep body, which is more or less ovate and rhomboidal in outline, while genera allied 

 to Pala?oniscus have the body much more elongated. The Acanthodini are fishes with small scales 

 like shagreen. Each fin carries a strong bony spine in front. Chiracanthus and Acanthodes have a 

 single dorsal fin, but Diplacanthus has two dorsal fins. These genera are chiefly found in the Old 

 Red Sandstone. The Dipnoal fishes are repi-esented in a fossil state by the Devonian genus Cteaodus, 

 and Ceratodus, known from teeth in the Trias and lower Oolites. The Chimseroid order does not range 

 further back in time than the Lias. It is represented by species of Ischyodus and Edaphodon in 

 Secondary strata, and by the genus Elasmodus, which is only known from Tertiary deposits. Fossil 

 Sturgeons from the Lias belong to the genus Chondrosteus ; in the London Clay the Acipenser toliapicus 

 is found. Of Sharks, the strata yield many remains, but they are chiefly known from teeth and the defences 

 which support the fins. The slug-like teeth of Acrodus, Strophodus, and Ptychodus, in the Secondary 



