ASTEROLEPIM). 69 



and extension in a plane which is nearly horizontal, and the extent of which is from 

 a position parallel and close to the side of the body to one at right angles to it. 



The distal portion of the limb, or lower arm, is more flattened, and shows a 

 dorsal and ventral surface, two sharp margins, external and internal, and a sharp 

 apex or point. It is composed of two central pieces (c), one dorsal and one 

 ventral, four marginals (m.), two external and two internal, and one terminal (t.). 



The elbow-joint is somewhat complicated. Each upper marginal 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 I should fancy 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 is covered with small, rounded, slightly imbricating 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-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 elongated 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 giving origin below to a fin-expanse 

 which is triangular in form, or slightly excavated behind, but not bilobate. In my 

 previous restoration of Pterichthys I omitted this fin, not having seen it at the 

 time, though I found it shortly afterwards in a specimen from Tynet in the 

 Manchester Museum. Meanwhile Mr. Smith Woodward discovered evidence of it 

 in a specimen in the British Museum, which he has figured in Part n of his 

 valuable ' Catalogue.' 



On the body and head there was a well-marked lateral sense-canal system, indi- 

 cated by grooves on the external surfaces of certain of the osseous plates, which 

 grooves have often been mistaken for sutures, especially those on the cranial 

 shield. On each side the lateral groove passes from behind forwards over the 

 posterior and anterior dorso-lateral plates, and thence on to the external occipital, 

 where it at once bifurcates, a transverse branch passing across the median occipital 

 to join its fellow of the opposite side. The main groove then runs forwards on the 



