PLACOGANOIDEI. 141 



for this form Professor Agassiz assigned the generic name 

 Pterichthys (ptcron, a wing, ichthys, a fish). Although, there- 

 fore, the term Aster olepis had been attached to a fragment of 

 the cuirass of this fish a few months previously, yet, as no 

 recognizable generic characters were associated with such 

 name, and as Aster olepis has been applied also to other 

 genera — e. g., Homosteus and Heterostius of Asmuss — the 

 example of British palaeontologists will be here followed, in 

 retaining the name Pterichthys for the present genus. " Of 

 all the organisms of the system," wrote the gifted Author of 

 the Old Red Sandstone, "one of the most extraordinary, 

 and the one in which Lamarck would have most delighted, 

 is the Pterichthys, or winged fish, an ichthyolite which the 

 writer had the pleasure of introducing to the acquaintance 

 of geologists nearly three years ago (1840), but which he first 

 laid open to the light about seven years earlier" (1833). 



Genus Pterichthys (fig. 59). — The head and the anterior 

 half of the trunk are defended by ganoid plates — i.e., plates 

 of hard bone coated with enamel ; those of the trunk forming 

 a buckler composed of a back-plate (fig. 59) and breast-plate 

 (fig. 60), articulated together at the sides. The rest of the 

 trunk was defended by small ganoid scales, giving it, like 

 scale-armour, flexibility. The fish bore a small dorsal fin (fig. 

 59, d), and a terminal heterocercal fin ; but these are very rarely 

 displayed in fossil specimens. The pectoral spines, c, are 

 formed of ganoid material, like the buckler. The armour of 

 the head, or helmet, z, io, appears to have been articulated by 

 a movable joint to the trunk-buckler u, 13. One of the few 

 existing ganoid fishes (Lepidosteus) is remarkable for the degree 

 in which the head moves upon the trunk. The component 

 dermal plates of the helmet correspond in some measure with 

 the position of the cranial bones in osseous fishes, but not 

 sufficiently to sanction the application to them of corresponding 

 names. They are indicated by figures in the cut 59 : 2 is the 



