Additional Note on the Pteraspidian Fish. 325 



Scaphaspis truncata (H. & S.), in the fine example figured 

 by Prof. Lankester (plate ii. fig. 3, Memoirs cited), and the 

 plate is similarly a little asymmetrical. 



The association of these two types of fish plates at several 

 localities may not be altogether without significance. Thus 

 in the Downton Sandstones of England there are Oyathas- 

 pis Banksii and Scaphaspis truncata. In the Onondaga varie- 

 gated shales of Pennsylvania, Prof. Claypole has found 

 Palceaspis* bitruncata and P. Americana.. So far as form 

 goes, it may be seen that the former is comparable with 

 the " dorsal scute " of Cyathaspis, as Prof. Claypole has 

 observed, while the latter in outline is not unlike the shield 

 of Scaphaspis. It should be remarked, however, that in the 

 course of the surface markings, as figured by Prof. Clay- 

 pole, Palceaspis Americana differs both from the Scaphaspis 

 of the Downton Sandstone and from plate d of our Acadian 

 species. 



Before closing this note, I may refer to a few other cha- 

 racteristics of the .Acadian fish. The plates c and d possess 

 the two ranks of stride which, accoi'ding to Prof. Lankester, 

 distinguish the Silurian from the Devonian Pteraspidians ; 

 the contrast between the larger set and the smaller inter- 

 mediate set of strife is more marked on plate d than on c ; 

 and the borders of both of these plates, as also the whole of 

 the lateral and rostral plates differ in having strise of uni- 

 form size. The superior prominence of certain of the strise 

 therefore belongs only to the two larger plates of the 

 dermal covering, but it is a useful character in distinguish- 

 ing these older fish from the more typical Pteraspids of the 

 Devonian system, in which no part of the dorsal scute pre- 

 sents these strikingly unequal stria? or ridges. 



The fineness of the strise in the plates of the Acadian fish 

 is quite up to the highest standard of tenuity in the fish 

 plates of Prof. Lankester's Memoir, there being from 150 to 

 200 of them in the width of an inch. The plate c is abun- 

 dantly dotted with minute pits apparently marking the 



* Quart. Jour. Oeol. Soe., London, Feb., 1885. This genus is sepa- 

 rated from the other Pteraspids on account of organic differences 

 in the structures of the plates, not because of difference in form. 



