ANTIARCIII. 



13 



most complex, ginglymoid articulation. Each appendage (ap.) is com- 

 pletely encased in closely-fitting plates and jointed once at about its 

 middle (J) ; while a large orifice in the supporting articular facette bears 

 witness to the passage of well-developed blood-vessels and nerves into its 

 interior. Near the hinder end of the posterior ventro- lateral plates there 

 is a distinct constriction with another inner transverse thickening, which 

 seems to mark the termination of the abdominal cavity. The tail is 

 comparatively small, covered with imbricating rounded or hexagonal 

 scales, with a series of large azygous ridge-scales on the dorsal aspect. 



FIG. 12. 



Pterichthys milleri (testudinarim) ; restoration by B. H. Traquair, from the 

 dorsal (A), ventral (B), and lateral (C) aspects, about one-half nat. size. 

 L. Old Red Sandstone ; Scotland, ap., pair of lateral appendages ; j, joint 

 in appendage; m., supposed upper jaw, with notches for narial openings; 

 op., operculum ; orb.; orbits ; double dotted lines indicating grooves for 

 sensory canal. (For names of dorsal and lateral plates, see lettering of 

 figs. 13 and 15.) 



The ridge-scales are soon interrupted by a small triangular dorsal fin, 

 which is membranous and only stiffened on its front margin by one, 

 two, or perhaps three bent scales; behind the fin they are very deeply 

 imbricating to the end of the tail, which is somewhat upturned, and is 

 bordered below by a large membranous caudal fin of uncertain shape. 

 The finest specimens of Pterichthys are obtained from the Lower Old Red 

 Sandstone of Scotland, but the genus also occurs in the Eifel, Germany. 

 The largest known example is about 0-16 m. in length. 



