DEVONIC FISHES OF THE NEW YORK FORMATIONS 39 



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

 cating osseous scales, which are arranged in longitudinal 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 different in shape from 

 those on the sides ; in front of the fin they are in the form of a few narrow 

 longitudinal median plates ; behind it they are elongated and imbricating, 

 like the fulcra or V-shaped scales along the body prolongation of the tail of 

 a Palaeoniscid fish. The dorsal fin is triangular, acuminate, and covered 

 with small scales, no distinct " rays " being seen ; and along its anterior 

 margin some prominent elongated scales are placed, producing an appear- 

 ance 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 giving origin 

 below to a fin expanse which is triangular in form, or slightly excavated 

 behind, but not bilobate. 



On the body and head there was a well marked lateral sensory canal 

 system, indicated by grooves on the external surfaces of certain of the 

 osseous plates, which grooves have often been mistaken for sutures, espe- 

 cially those on the cranial shield. On each side the lateral groove passes 

 from behind forwards over the posterior and anterior dorsolateral plates, 

 and thence on to the external occipital, where it at once bifurcates, a trans- 

 verse branch passing across the median occipital to join its fellow of the 

 opposite side. The main groove then runs forward on the lateral cranial 

 plate, and arriving in front of the median opening it bends inwards to join 

 the opposite groove on the premedian plate ; a slightly different arrange- 

 ment, however, is seen in Bothriolepis, as we shall see when that genus 

 comes to be specially considered. 



Genus astbboi.epis Eichwald 

 Definition. Median dorsal plate overlapping both the anterior and 

 posterior dorsolateral plates, Arms shorter than the system of body plates ; 

 articular plates of the upper arm scarcely meeting externally over the 



