328 



ORDERS OF FISHES. 



Of the Glyptodipterines with cycloidal scales, the most 

 important form is Holoptychius (fig. 288). In this genus 

 there are two dorsal fins placed very far back, and the 



. rtj.Glyptolcemus Kiitnairiii. Restored. A. Scale of the same. Devoniar 



Fig. 288. Holoptychius nobilissimus. Restored. A, Scale of the same. Devonian. 



ventrals are similarly approximated to the tail, as in Glypto- 

 /cemus. The body, however, is covered with large scales of a 

 cycloidal form, which overlap one another, and have wrinkled 

 surfaces ; and the tail is inequilobed. The teeth are of two 

 sizes, and the larger ones are longitudinally striated at their 

 bases. The true Holoptychii are Devonian in their distribu- 

 tion. In the Carboniferous Rocks, however, occurs the nearly 

 allied genus Rhizodus (fig. 289), in which the teeth agree with 

 those of Holoptychius in being of two sizes, but they differ in 

 being trenchant on both sides. Rhizodus must have attained 

 a large size, and must have been highly predaceous in its 

 habits. 



The type-genus of the Ctenodipterini is Diptcrus, in which 

 the position and form of the fins are very much the same as 

 in Holoptychius. The body is also covered with cycloidal 

 overlapping scales ; but these scales are smooth. The head 

 is protected by a kind of helmet formed of the anchylosed 



