198 



P TERA SPIDOMORPHI 



The dorsal and the ventral edge of the tail are strengthened with a 

 median series of stout fulcral scales (Traquair [467-8]). 



Drepanaspis, Schliiter ; Lower Devonian, Prussia. 



Family PSAMMOSTEIDAE. Incompletely known fossils, represented 

 by large convex plates, isolated fragments apparently the posterior 

 corners of the lateral flaps, scales, and fulcra. All these are covered 

 with an outer layer of denticles, often with closely fitting crenulated 

 edges, strikingly like those of Thelodus. The denticles are fixed on an 



Fio. 160. 



Restoration of Drepanaxpis geniiitideneiisis, Sclil. Dorsal (A) and ventral view (B). Lower 

 Devonian, Bundenbaoh. (After Traquair, from Brit. Mus. GuMe.) a.v.l, anterior ventro-lateral ; 

 e.l, external labial ; m, mental, behind the mouth ; m.d, median dorsal ; m.w, median ventral ; 

 p.l, postero-lateral ; p.v.l, posterior ventro-lateral; r, rostral; x, orbital plate with orbit; 

 cloacal aperture behind the median ventral plate. 



underlying layer of bony substance with large vascular spaces (Fig. 

 166, D and H). The plates are frequently marked with superficial 

 polygonal areas, somewhat like those of the Cephalaspid armour 

 (Traquair [466]). 



Psammosteus, Ag. ; Devonian, Europe. 



Family PTERASPIDAE. The head and body are narrower, and the 

 lateral flaps are but little developed (Fig. 171). The exact shape of the 

 tail is unknown, but there is reason to believe that it was heterocercal. 

 No dorsal fin has been observed (Lankester [276], Traquair [466], 

 Drevermann [122], Alth [19a]). 



The best-known genus, Pteraspis, has the entire dorsal region of the 

 head and body covered by a bony shield, projecting into a rostrum in 



