Geology. | 139 
they are convex and trilobed, with only the narrow middle lobe trans- 
versely sutured, the lateral being broad and smooth; it is called Pemphi- 
gaspis bullata. Professor Hall observes with regard to the distribution 
of the species of the Upper Mississippi valley : 
“ Although I have not been able to recognize the successive Trilobite 
beds of the Sandstone as indicated by Dr. Owen, I can nevertheless refer 
ed to 
s of the formation, and clearly separated from the great central mass, 
we have the Genera Dicellocephalus, Triarthrella and Aglaspis, together 
with Lingula, Serpulites and Euomphalus. We observe, therefore, that 
the earliest trilobites are referable to the genus Conocephalites; and the 
genus Dicellocephalus does not appear in the first stages of the formation, 
stages of the sandstone, that the typical species of this genus of Dr. 
Owen appear; and those from the lower m, 
belong apparently to other genera.” * * ‘ 
; tages recognized, the physical con- 
ditions have been very monotonous hie the entire period; and in 
ar indications. We find great 
h 
trilobi - 
[We may here remark that the spelling Dikelocephalus, although it is 
that of Owen, the author of the genus, is wrong, and no authority can 
