74 Dr. F. V. Hayden on the Primordial Sandstone 
Arionellus? Oweni, (M. and H.) Proc. Acad. Nat. Sci. Philad. 
The only specimen of this species we have 4, 
seen is a cast, retaining none of the shell, and — 
of course giving no idea of the nature of the 
external markings if there are any. Nor is it 
in a condition to enable us to determine wheth- ; il | am) | 
er or not the posterior lateral extremities of the “—— a 
buckler are pointed though they appear to be. Ata point nearly — 
opposite the middle of the glabella, there is on each cheek 
less than half way down the slope from the furrow between the 
check and the glabella, what appears to be some remains of sm 
eyes, though the specimen being unfortunately a little defective 
ere on both sides, the nature of these prominences cannot be 
clearly made out. 
As near as can be determined from a shadowy medal ruled 
figure of an imperfect specimen, this Trilobite seems to be nearly 
related to a form represented by Dr. Owen on plate 1, A (fig. 
13) of his Report on the Geology of Wisconsin, Iowa and Minne- 
sota under the name Crepicephalus ; though the anterior margil 
of its head is more narrowly rounded, and its glabella less taper- 
ing. Until specimens showing its facial sutures can be obtained, 
its generic relations will have to remain somewhat doubtful. . 
ear the central portion of the Black Hills, and at the Big 
Horn Mountains.* 
We have now described this member of the Primordial zone 
as far as it has occurred within the limits of our own observa 
tions. It now becomes an interesting point to determine its geo 
graphical extension in the West, and for that purpose we propose 
to summon all the evidence at our command. ‘The proof will 
not, however, be as satisfactory as could be desired owing to the 
general absence of organic remains. 
If we now extend our examinations far north into the Hud 
son’s Bay territory we find that much interesting information has 
been obtained in regard to the Silurian rocks of that region, but 
not accompanied by the evidence which gives to the knowledge 
acquired that definiteness which is desirable. It is probable, 
however, that when not eroded away or concealed by more Ie 
cent caf aa the Potsdam sandstone and perhaps rocks of more 
recent Silurian age occur all along the margins of the Roc 
Mountains to the Arctic Sea. To what extent still more re 
cent or Upper Silurian occur over this vast region, our present 
knowledge will not enable us to determine, but the few fossils 
which have been collected indicate that the great Silurian Sea 
over much of the northwest. Sir John Rich 
* efer in : of 
ine Mimeitu.an oe in the Museum © 
ed 
Smear IeN 
r 
