82 IOWA GEOLOGICAL SURVEY 



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, 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 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 arrangement, however, is 

 seen in Bothriolepis, as we shall see when that genus comes to 

 be specially considered. 



Genus ASTEROLEPIS Eichwald. 



Median dorsal plate overlapping both the anterior and pos- 

 terior dorso-lateral plates. Arms shorter than the system of 

 body plates ; articular plates of the upper arm scarcely meeting 

 externally over the external marginal; forearm consisting of 

 two centrals, two pairs of marginals and a terminal, the latter 

 forming the pointed extremity of the limb. Postmedian plate 

 of the head large and broad, excluding the median occipital from 

 the margin of the orbit ; premedian plate notched in front ; extra- 

 lateral plate loosely articulated with the rest of the cranial shield. 

 Superficial ornament consisting of raised tubercles with stellate 

 bases, which may be sometimes confluent. Lateral line system 

 on the head connected by two commissural canals, a posterior 

 one crossing the hinder part of the median occipital, and an 

 anterior one crossing the premedian plate in front of the orbit. 

 Tail unknown. 



The characters of this genus do not differ materially from 

 those of Pterichthys, so far as known, except as regards the 

 mode of articulation of the anterior median dorsal plate, and 

 in the number and arrangement of the plates of the pectoral ap- 

 pendage. No specimens of Asterolepis have yet been discov- 

 ered, however, which show the tail. Like Bothriolepis, this 



