970 



CLASS PISCES. 



lobate ; while their internal skeleton forms a shortened uniserial 

 archipterygium. The teeth are labyrinthodont in structure, lacking 

 the complex internal network found in the preceding family. The 

 scales are cycloidal and sculptured ; the median fins are numerically 

 the same as in Holoptychius ; but the tail approxi- 

 mates to the truly diphycercal type. The lower 

 jaw has an inner series of tusk-like teeth (fig. 

 905). As an imperfectly known form we may 

 first mention Cricodus (Polyplocodus) from the 

 European Devonian. In the well-known Jristi- 

 chopte?-us, of the Scottish Devonian, the body is 

 much elongated, the tail distinctly heterocercal, 

 the posterior fins are placed near the tail, the 

 opercular bones are unusually large, and the 

 scales thin and striated. An apparently allied 

 form is Eusthenopteron of the Devonian of Can- 

 ada, in which the vertebral centra appear to have 

 been unossified, while there are slight differences 

 in the interspinous bones, tail, and teeth. Ac- 

 cording to the views of Dr Traquair, Gyroptychius 

 (fig. 904), from the Devonian of Scotland, is 

 allied to Tristichopterus, having scales of the 

 same general type (although some of these were 

 formerly thought to be rhomboidal), but a more 

 rhomboidal tail, and the exposed portion of the 

 scales relatively larger. From the Carbonifer- 

 ous of both Europe and North America we have 

 the type genus Rhizodus (Megalichtkys in part), 

 of which the typical species attained a very large 

 size. The larger teeth (fig. 905), on the evidence 

 of which the genus was originally founded, have 

 the upper half of the crown smooth, while the 

 lower half is longitudinally fluted. The teeth of 

 large individuals (which may attain a length of 

 nine feet) are nearly two inches in height. In 

 Rfa'zodopsis, of the same deposits, the cranial 

 bones form a well-developed shield on the supe- 

 rior surface resembling that of the Osteolepididce, 

 although the mandible agrees in general structure 

 with that of Rhizodus, and consists of articular, 

 angular, and dentary elements ; the latter having 

 a series of infra-dentary pieces on its inner side 

 which carry the large tusk-like teeth. The inferior space between the 

 mandibular rami is occupied by a series of jugular plates, of which 

 there is a large median pair, together with a small unpaired anterior 



Fig. 904. — Gyropty- 

 chius angustus", from 

 the Devonian of Scot- 

 land. Reduced. (After 

 Pander.) 



