OP THE AMEEICAN PTEEASPIDIAN, PAL^ASPIS. 543 



taken to represent as many species, to which respectively the names 

 Palceaspis americana and P. hitruncata were applied. The former 

 was usually rather the larger of the two and was rounded at one 

 end, if not also at the other. The latter, as the name implies, was 

 bitruncate and especially square- cut at one extremity. 



The microscopic structure of these shields, which was fully 

 described in the original paper above referred to, sufficiently proved 

 the affinity of the specimens, agreeing as it did in every respect 

 with that of the shield of Pteraspis as described by Huxley in his 

 memoir on the subject in this Journal,^ and by Lankester in his 

 monograph on the fishes of the Old Eed Sandstone, published by 

 the Palaeontographical Society.^ 



The above-mentioned papers constituted the bulk of what was 

 accessible to the writer at the time of publishing his original 

 description, now seven years ago. The papers of Kunth and of 

 Schmidt had, it is true, appeared, but they were not at the time 

 accessible to him, and no other data available for the purpose were 

 within his reach. During this interval oar acquaintance with the 

 family has considerably advanced, especially with regard to the 

 American forms. Sufficient time has been given for working out 

 some more of the old Pennsylvanian material, and also for obtaining 

 a certain amount of new specimens. In addition to this, at leasit 

 one new species has been described on the American continent and 

 the epoch-making paper of Yon Alth has appeared in Austria- 

 Hungary. By these various additions to the literature of the subject 

 we are placed in a position to better understand the structure and 

 economy of the strange fishes of the early seas. 



II. Kunth's Discoveey 01" Pteraspis integea. 



It will be desirable before proceeding farther with our subject 

 to pass briefly in review the progress of our knowledge concerning 

 this ancient ichthyic fauna. Twenty years ago three forms of 

 Pteraspidian fishes had been recognized in Europe. These were first 

 clearly defined as genera by Lankester in the monograph already 

 cited. For one of these he retained the original name Pteraspis, 

 given by Kner to his specimen, though in misapprehension of its 

 real affinity. To the other two he applied the names Cyaihaspu 

 and JScaphaspis, suggested by their forms. This was the commence- 

 ment of real progress in the systematic arrangement of these 

 fossils. 



The minute structure of these shields had been studied some 

 years previously by Huxley in the memoir already alluded to, and 

 this structure has been ever since the crucial test, by which almost 

 the very smallest fragment of a Pteraspidian may infallibly be 

 recognized. 



All these three fossils were considered by Lankester in his mono- 

 graph as representing the dorsal plates of as many distinct genera 



1 Quart. Journ. Geol. Soc. vol. xiv. (1858) p. 274, &c. 

 ^ Palseontogr. Soc. Monogr. 1868. 



