72 (Bulletin of the (Natural History Society. 
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 Cyathaspis 
Banksii and Scaphaspis truncata, and in the Onondaga varie- 
gated shales of Pennsylvania, Prof. E. W. Claypole has found 
Palceaspis * bitruncata and P. Americana . So far as the form 
indicates a relationship, it may be said that the former is 
comparable with the dorsal scute of Cyathaspis , as Prof. 
Claypole has observed, while the latter is not unlike the shield 
of Scaphaspis in outline. It should be remarked, however, 
that in the course of the surface markings as figured and 
described by Prof. Claypole, Palceaspis Americana differs both 
from the Scaphaspis of the Downton Sandstone and from the 
ventral plate of our Acadian species. 
In one respect the Acadian Petraspid is decidedly of arch- 
aic type. The plates c and d possess the two ranks of striae, 
or ridglets, which, according to Prof. Lankester, distinguish 
the Silurian from the Devonian Petraspidian fishes; the con- 
trast between the larger and the smaller intermediate set of 
ridges is more marked on d than on c, and the borders of 
these plates, as also the whole of the lateral plates and the 
rostral plate, differ in having the ridges of uniform size. The 
superior prominence of certain of the strias in our species 
(and probably in others) belongs therefore only to the two 
larger plates of the dermal covering, but it is a useful char- 
acter to distinguish these older fishes from the more typical 
Pteraspids of the Devonian Age, in which no part of the 
dorsal scute presents these strikingly unequal ridges or strife. 
The fineness of the ridglets on 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 abundantly dotted with minute pits appar- 
ently marking the sites of mucous glands, which Prof. Lan- 
kester mentions as a feature of the shields of Pteraspis: from 
this he infers that a secreting membrane probably covered 
* Quart. Jour. Geol. Soc. London, Feb. 1881, This genus is separated from the 
other Pteraspidian fishes on account of organic differences in the structure of the 
plates and not because of difference of form. 
