HISTORICAL REVIEW. 345 



trunk region of Pteraspis, for these scale-like structures were regarded as conclu- 

 sive proof that the pteraspidae belong to the vertebrates. He says (1868, page 

 18) "All that is known as regards the scales of these fishes is from a single 

 specimen found in the cornstones of Herefordshire." This specimen, he says 

 elsewhere (1873, page 191) "Shows seven rows of rhomboidal scales attached 

 (not merely adjacent to) to a portion of the head shield of Pteraspis. That these 

 are true scales, or lozenges of sculptured calcareous matter is absolutely certain. 

 It is also absolutely certain that the shield is pteraspidian and that the scales and 

 shield belong to the same individual organism. The scales are fish-like. I know 

 no arthropod, nor any other organism except a fish which possesses any structure 

 even remotely representing them." "The shields of the chitonidas and cerripeda 

 are the only animal structures, except the scales of a ganoid fish (with which they 

 agree exactly) which they could even vaguely suggest." "The form of this 

 shield, and its details as to apertures, processes, etc., agrees with the view that it 

 belongs to a fish most fully. It has not the remotest suggestion of crustacean 

 affinities about it." 



After commenting on the fact that the fossil in question was marked with 

 long parallel striae, and that the middle layer contained the polygonal cavities, he 

 adds (1864, page 195), "This structure, which has no parallel among fishes, or, 

 indeed, any group of the animal kingdom, leaves no possibility of a doubt that 

 the specimen is a fragment of Pteraspis." Lankester further maintains (1868, 

 page 4) that by the discovery of these scales "the piscine nature of these fossils 

 was definitely set at rest." 



These positive statements are contradictory and are hardly warranted by 

 the facts, for the crustacean character of the shields had been repeatedly com- 

 mented on by competent observers, and in his own monograph (page 61) he has 

 described a fragment, possibly connected with Cephalaspis which he names 

 Kallostrakon podura (Tolypaspi ?) "on account of the resemblance to the well- 

 known microscopic markings of the scales of the insect Podura." 



But all recent students of the shell of Pteraspis are agreed that it is not " ex- 

 actly " like that of a ganoid fish, in fact its microscopic structure is altogether of a 

 different character, and it is not true that there are no arthropods with structures 

 even remotely resembling the scales of Pteraspis, because in Pterygotus the entire 

 body is covered with an ornamentation astonishingly like fish-scales in outward 

 appearance, so much so as to deceive such a keen observer of fishes as Louis 

 Agassiz. Moreover, in many trilobites, in the ceratiocarina, and in arachnids, 

 (Phrynus), the surface of the shell is ornamented with ridges and grooves not 

 unlike those of Pteraspis in external appearance. 



Probably neither Hux'ey nor Lankester would have made the above state- 

 ments had they kept Pterygotus in mind, or had they been acquainted with the 

 structure of the shield of Limulus. 



In 1889, the author compared the arrangement of plates and sense organs 

 in the cephalic buckler of Cephalaspis and Pterichthys with those on the cephalo- 



