262 Evolution and Distribution of Fishes 



The Cephaloplacoda (Cephalaspldae) include five 

 genera, Cephalaspis (Fig. 9a, p. 119) Etikeraspis, Auche- 

 naspis, Didymaspis, and Tremataspis, that extended from 

 the Silurian upward to Lower Old Red rocks, and that cover 

 a territory nearly coterminous with the Symplacoda. 



The Polyplacoda (Antiarchi) includes the four genera 

 Pterichthys, Bothr'wlepis (Fig. 11, p. 124), Asterolepis, and 

 Microbrachius that form a remarkable assemblage with 

 evident affinities to each other, but that are highly aberrant 

 in relation to the main lines of fish evolution. Beginning 

 with Pterichthys, that is abundant in the Lower Old Red 

 rocks of N. Scotland and has often been figured in text- 

 books and memoirs, species pass into the Upper Old Red 

 or Devonian of W. Russia, of Germany, of Britain, of 

 Scaumenac Bay in Canada, and of Pennsylvania. It is sug- 

 gestive also to observe that the eight species of Bothri- 



FlG. 41 



Fig. 42. 



Fig. 41. Birkenia elegans, a primitive Silurian fish, found abund- 

 antly in Downtonian beds of Lanarkshire, Scotland; slightly en- 

 larged, cf, caudal fin; df, dorsal fin; lo, lateral and probably 

 branchial pores; o, eye centre; vs, ventral scutes. 



Fig. 42. Lasaniiis problematlcus, found with above, and probably 

 closely allied; slightly enlarged. (Both after Traquair). 



