DEVONIAN FISHES OF IOWA 81 



trally this part of the limb is completed by the dorsal and ventral 

 anconeal pieces (d. a. and v. a.). 



The distal portion of the limb, or "lower arm", is more flat- 

 tened, and shows a dorsal and ventral surface, two sharp mar- 

 gins, external and internal, and a sharp apex or point. The 

 ventral surface, and presumably the dorsal as well, is com- 

 posed of two central pieces (c), and six marginals (w), three 

 of which are external and three internal. It is the distal mar- 

 ginal on the inner side which forms the acute point or apex of 

 the appendage, as first noticed by Jaekel.* 



The elbow-joint is somewhat complicated. Each upper mar- 

 ginal of the lower arm is furnished above with an articular 

 process, which is received within the lower extremities of the 

 internal and external marginal plates respectively of the upper 

 part of the limb. Then, on the other hand, each anconeal plate, 

 dorsal and ventral, of the upper arm has a small, flat articular 

 process below, which fits into a slit on the outer surface of the 

 upper extremity of the corresponding upper central of the lower 

 arm. It is hard to say how much movement could have been 

 here allowed, but from the form, of the joint it seems probable 

 it was limited to a slight flexion and extension, and possibly only 

 in the horizontal plane, as in the case of the shoulder. 



The tail (in Pterichthys) is covered with small, rounded, 

 slightly imbricating osseous scales, which are arranged in longi- 

 tudinal rows, and also in transverse bands. On the dorsal aspect 

 behind the carapace there is a small median fin. Along the 

 dorsal margin the scales are in the form of a few narrow longi- 

 tudinal median plates; behind it they are elongated and imbri- 

 cating, like the fulcra or V-scales along the body prolongation 

 of the tail of a Palseoniscid fish. The dorsal fin is triangular, 

 acuminate, and covered with small scales, no distinct "rays" 

 being seen ; and along its anterior margin some prominent elon- 

 gated scales are placed, producing an appearance which has been 

 mistaken for a spine. The hinder extremity of the fish is formed 

 by a completely heterocercal caudal fin, the body axis curving 

 upwards as a pointed and slightly arched prolongation, and 



* Jaekel, 0., TTeber die Organisation und svstematische Stellung der Asterole- 

 piden. Zeitschr. deutsch. geol. Ges., Mai-Protokoll, 1903, 53, pp. 41-60. 



