OF THE AMERICAN PTERASPIDIAX, PALiEASPIS. 557 



dispels all such expectation, and shows in a moment that this state- 

 ment is only another of the misapprehensions into which this wTiter 

 was led in consequence of his mistaken view of the fossil. Regarding 

 it as a roUed-up trilobite he was puzzled to explain the side-pieces 

 which were visible on his slab between the head- and tail-shields of 

 his ' crustacean.' He could not compare it with any known part of 

 any known species, nor was it probable that any unknown form 

 would differ very widely from these. He consequently was driven 

 to the view that his fossil represented some not yet discovered 

 family, and that these side-pieces were the remains of locomotive or 

 feeding organs. But in the light of the now known structure of 

 Palceaspis it is evident that Kunth was dealing with a somewhat 

 similar form, wherein the side-pieces beiiig rather loosely anchy- 

 losed to the dorsal shield fell away when the soft retaining-parts 

 deca3'ed. The' close corres^Dondence of these parts with the ' cornua ' 

 of Gyathaspis should, one would think, have almost sufficed to 

 prevent the complete misinterpretation which led Kunth astray in 

 regard to his fossil. The insertion of such organs in a crustacean 

 of trilobitic affinity is hardly less foreign to what we know of the 

 structure of the group than would have been the addition of lateral 

 pieces, and scarcely aided the case or lessened the difficulty. 



IX. Comparison of Paljeaspis with other Pteraspids. 



One diiference, however, between Kunth's fossil and Palceaspis is 

 obvious from an examination of his figures. It is that in the former 

 the side-pieces lie in front of the mid-point, whereas in the latter 

 they lie behind it. There is no possibility of otherwise explaining 

 either Kunth's figure or my specimens. This may of course be due 

 simply to difference of species or rather of genus, for so great a 

 variation must be more than specific. But our present knowledge 

 scarcely warrants a definite resort to this explanation. If, however, 

 the side-pieces in Kunth's specimen really lay in life as represented 

 in his figure, and of this I can see no reasonable doubt, then it 

 would not be possible to include it in my genus. But there is a 

 possibility which may one day remove this existing discrepancy. 

 Though no specimen in my possession shows an antero-lateral plate 

 in position, or any convincing traces of such an arrangement, yet 

 from some indications I should not be surprised if such a case should 

 come to light and the species should be found to have had side-pieces 

 in front similar to those of Kunth's figure. This would render the 

 armour of the animal more symmetrical and in harmony with that 

 of some other members of the family, notably Cyathaspis, in which, 

 however, the two plates are represented by a single one, and the 

 critical point where a fin may have been attached in Palceaspis is 

 not apparent. Moreover, the hinder part of Kunth's specimen is so 

 badly broken that it is in my opinion impossible to say that it did 

 not originally possess a postero-lateral plate resembling that of Palce- 

 aspis, and lying in a corresponding position. Should this prove to 

 be the case, the two would very closely correspond in structure, 



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