THE AMERICAN NA TURALIST. [Vol. XXX VII. 



among the true fishes, and their existence practically ignored. 

 Although the pteraspids are now generally placed among the 

 true fishes, their head shields do not present a single recogniz- 

 able vertebrate character. The various surface markings have 

 been supposed to indicate the presence of median or lateral eyes, 

 olfactory, or auditory, organs, or gill openings, or the impres- 



resemblance to corresponding organs in true vertebrates, but 



which they can be compared. There are no traces in these 



lower jaws, fins, notoehord, or vertebral column. 



The genus Pteraspis was first proposed by Rudolph Kner in 

 1847 to include the forms described in 1835 by Agassi/ as 

 Ccphalaspis lacisii* and C. lloydii. Their appearance was so 

 unlike the ordinary fish remains that for a long time Kner did 

 not suspect that they had been already described by Agassiz 

 in his Poissons Fossils. 



From a study of their minute structure Kner believed them 

 to be the internal shells of cephalopods allied to Sepia. 



In 1856, F. Roemer described a form closely related to C. 

 lloydii as Palaeoteuthis, and referred it to the Sepiidae, but sug- 

 gested that the forms described by Kner were crustaceans 

 related to Uithyrocaris or Pterygotus. 



In 1864, Lankester divided the Pteraspidae into the three 

 genera, Pteraspis. Cyathaspis and Scaphaspis. But in 1872, 

 Kunth described a shield of Cyathaspis, below which he found 

 one belonging to Lankester's genus Scaphaspis, and he rightly 

 concluded that the two shields belonged to the same animal. 

 I Ie maintained that the lower shield bore the same relation to the 

 upper one that the tail plate of a rolled up trilobite does to its 

 head shield, and that between the two were a number of pieces 

 comparable with the segmental trunk plates of a trilobite. Other 

 plates were present which Kunth regarded as locomotor organs, 



ring to Huxley's statement that there is no molluscan or crusta- 



