HOLOPTYCHIOID FISHES. 65 



In the family Holoptychiidae, e. g. Holoptychius flemingi piolo- 

 (186, and fig. 36), the remains of which fish occur in the ptychius. 

 Upper Old Red Sandstone of Scotland and the Upper Devonian 

 of Belgium, the body is covered with overlapping cycloid 



Fig. 36.— Holoptychius flemingi, restored. 



scales with a superficial layer of hard substance resembling the 

 enamel of teeth, and called vitro-dentine or ganoin. The pectoral 

 fins have a long, pointed, scaled a xis or lobe, the vertebral 

 column has no bony centra, the axis of the tail is slightly up- 

 tilted, and the vertical infoldings of the teeth are very numerous 

 and complex. 



In the family Rhizodontidse, represented in the exhibited series 

 by Eusthenopteron foordi (188, and fig. 37), a fish found in 

 the Upper Devonian of Canada, the scales are* cycloid, like 

 those of Holoptychius, the pectoral and pelvic fins are shorter 

 than those of the Holoptychian fishes, the vertebral column 

 has ring-like centra in some of the genera, the axis of the tail 

 is either straight or is slightly uptilted, the teeth are conical, 

 and the vertical infoldings of the walls are comparatively 

 simple. 



Resembling the Rhizodontidse in the pectoral fins being obtusely 

 lobate, but differing in the scales being rhomboidal instead of 



F 



