OSTEOLEPIDOTI 2S5 



vertebrae appear never to have been ossified. The notochord was possibly 

 unconstricted. The scales are cycloid, and deeply overlapping ; their 

 exposed surface covered with a modified cosmoid layer sculptured in 

 ridges and knobs (Rohon [369]). The teeth acquire a marvellous com- 

 plexity of structure owing to the infolding of the wall, especially near 

 the base ; it is owing to this feature that the family has been named 

 Dendrodontidae (Owen [311rt, 368, 453, 462]). 



Glyptolepis, Ag., and Holoptychius, Ag. (Fig. 254) ; Devonian, Europe 

 and America. Dendrodus, Owen ; Devonian, Europe. 



Family Glyptopomidae. Like the preceding family ; but the paired 

 fins less acute, the tail diphycercal, and the scales mostly rhomboidal 

 and only slightly overlapping, though sculptured. There is a pineal 

 foramen [228]. 



Glyptopomus (Glyptolaemus), Ag. (Fig. 254) ; Devonian, Europe and 

 North America. 



Family Osteolepidae. The scales are rhomboid, and, like the cranial 

 plates, have a thick layer of shiny cosmine. There are large strong 

 scales at the base of the fins, and a 

 median anterior gular. The cranial uf. 



roof bones are often fused together, \ aX.. 



especially in front. A pineal foramen 

 is preserved, except in Megalichthys. 

 The teeth are but little folded. The 

 paired fins have a short and some- 

 what obtuse lobe. The tail may be 

 very slightly heterocercal (hetero- 

 diphycercal) as in Diplopterus (Fig. 

 255), or quite heterocercal as in FlG - -'"'"'• 



Osteolepis (Fig. 253) [312, 227-8]. Tail of Diplopterus Agassigii, Traill. (After 



* 7 • -17- i ,-£>• OKO \ nv Traquair.) ax, extremity of axis ; a.f, anal 



Osteolepis, Val. (Tig. 253) ; lliur- fin ; c/, hypochordal fin ; d./, dorsal fin. 



sius, Traquair j Diplopterus, Ag. 



(Fig. 255) ; Devonian, Scotland. Megalichthys, Ag. ; Carboniferous and 



Lower Permian, Europe and North America. 



Family Rhizodontidae. The scales overlap deeply, and are gener- 

 ally thin, cycloid, sculptured, and without cosmine. A shiny cosmine 

 layer is, however, preserved in Gyroptychius, in which genus the scales 

 are somewhat rhomboid. The caudal fin is diphycercal or hetero- 

 diphycercal (Tristichopterus). Usually there are three well-marked lobes, 

 into the middle one of which projects the extremity of the notochord 

 (Figs. 253 and 258). The paired fins are obtusely lobate, and not strictly 

 mesorachic ; the fin-lobe is somewhat triangular and the anterior edge is 

 strengthened (p. 282). There is in some a small median anterior gular 

 plate, and the teeth are considerably folded at their base. The endo- 

 skeleton is generally well ossified, bony, ring-like centra being frequently 

 present [488, 445, 450]. 



It is evident from the structure of the paired fins that the Rhizodonts 

 differ considerably from the previous three families, and they should 

 perhaps not be included in the same Order (Figs. 244, 252). 



Gyroptychius, M'Coy ; Tristichopterus, Eg. (Fig. 253); Devoijian, 



