ORDERS OF FISHES. 



143 



Of the Glyptodipterini with rhoinboidal scales, GlyiJtolcemus 

 (fig. 506) may be taken as the type. In this singular fish, 

 the body is elongated and the head depressed. There are two 

 dorsal fins which are placed very far back, and the ventrals 

 have a similar posterior position. The tail is " divided into 

 two equal lobes by the prolonged conical termination of the 

 body," becoming thus " diphycercal." Glyptola^mus is ex- 

 clusively confined to the Devonian period. 



^^^^^^^^^W^^^r 



Fig. 506. — GlyptoUemus Kinnairdi. Restored, a, Scale of the same. Devonian. 



Of the Glyptodipterines with cycloidal scales, the most 

 important form is Holoptychius (fig. 507). In this genus 

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



Fig. 507. — Hohptyclmts nobilissinms. Restored, a, Scale of the same. Devonian. 



trals are similarly approximated to the tail, as in Glyptolcemus ; 

 while the pectorals are similarly acutely lobate. 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 Holoptycliii are Devonian in their distribution. In 



