Fig. 192. 



110 PISCES class i 



Family 1. Gymnodontidae. Olivier. 



Trunk short and deep, naked or covered with bony spines. Jaws beak-shaped, 

 with a cutting dental plate above and below, either undivided or in right and left 

 halves. No dorsal spines. Eocene to Recent. 



Fossil remains of this family are very rare. Large jaws of Orthagoriscus 

 have been found in the Oligocene of Belgium. Dioclon occurs in the Eocene, 



Oligocene, and Miocene ; Gymuodus in the Miocene ; 

 Heptadiodon in the Upper Eocene of Monte Postale. 



Family 2. Sclerodermidae. Cuvier. 



Jaws with a small number of separate teeth. Skin with 

 scales or roughened. Dorsal spines usually present. Eocene 



Pharyngeal teeth. A, An- to Recent. 

 eistrodon libycus, Zitt. Upper „. .. „ i -n ?• . /t» » 



Chalk; Gasr Dachi, Libyan 1 he existing genera Ustracion and Batistes (rroto- 



(?,mr5S;Ge 7 rVafs^p. iS Eocene'; oalistum, Massal.) have representatives in the Upper 

 Mukattam, near Cairo (after Eocene of Monte Bolca. The extinct genera Acantho- 



Daines). 1/77 



derma and Acanthopleurus, Ag., occur in the Upper Eocene 

 black slates of G-larus. The teeth described as Ancistrodon, Roemer (Fig. 

 192), from the Upper Cretaceous, Eocene, and Oligocene, may belong, at 

 least in part, to the pharyngeal dentition of Scleroderms. 



Range and Distribution of Fossil Fishes. 



Notwithstanding the apparently favourable circumstances for the preserva- 

 tion of fishes due to their aqueous habitat, their geological history is still very 

 imperfectly known. Complete skeletons, it is true, are rather numerous in 

 clayey, calcareous, or marly shales, which were laid down as fine mud on the 

 bottom of former lakes and near the shore in seas. On the other hand, in 

 rocks of coarser grain (sandstone), in very many shore deposits, and also in 

 deep-sea limestones, there are usually only isolated teeth, scales, dermal 

 plates, vertebrae, scattered bones of the skeleton, and otolites, which are 

 extremely difficult to determine. In very many marine, lacustrine, and 

 fluviatile deposits, fish remains are almost completely wanting, so that the 

 formations rich in fossils are usually separated from each other by a series of 

 strata which represent long periods of time. 



The oldest undoubted traces of fishes both in Europe and North America 

 occur in rocks of Silurian age. They are found in the Ludlow Bone-bed, in 

 the light, dolomitic, fissile limestone of the island of Oesel in the Baltic, and 

 in sandy shales in Podolia and Galicia. They are also found in the Onondaga 

 Group of Pennsylvania. The determinable forms are all Selachii and primi- 

 tive Ostracodermi (Coelolepidae, Pteraspidae, and Cephalaspidae). 



In the Devonian era fishes begin to attain a great development, and are 

 sometimes discovered in a remarkable state of preservation, especially in the 

 Old Red Sandstone of Great Britain, the Russian Baltic provinces, Podolia, 

 and Galicia, and in the corresponding formations of North America. Scattered 

 fish remains, such as plates of Ostracodermi and spines of Selachii, also occur 

 in the uppermost stages (F, G) of the Silurian basin of Bohemia, and in the 



