638 THE AMERICAN NATURALIST. [Vok XXXII. 
Precambrian Classes. Cambrian Classes. 
Rhizopoda (Radiolaria). Rhizopoda (Foraminifera and Radi- 
olaria). f 
Porifera (Hexactinellid Sponges). Porifera (Sponges). 
Hydrozoa (Medusz and Graptolites). 
Actinozoa (Corals). Actinozoa (Corals). 
Brachiopoda. Brachiopoda. 
Annelida. Annelida. 
Crinoidea 
Asteroidea. 
Lamellibranchiata. 
Mollusca, Gastropoda (including Pteropoda).' 
? Cephalopoda (Orthoceras ?). 
Trilobita. l Trilobita. 
Crustacea. 
It would seem from these data that the physical condition of 
the sea and atmosphere was favorable to the existence of types 
for aught we know quite or nearly as highly specialized as those 
of the same classes now in existence. Life and nature in the 
Precambrian went on, so far as we can tell, much as in Cambrian 
times. Though locally there are breaks in the continuity of 
geological processes, yet probably over the world generally 
there was a continuity of geological phenomena, and on the 
whole a tolerably unbroken series of organic forms. 
It is obvious, however, that in the regions thus far examined, 
the Precambrian, whether we include the Archæan or not, more 
than at any time since, though the land areas are by some con- 
sidered to be of small extent, was a period of widespread and 
profound changes in the distribution of land and sea. While it 
is generally supposed that the extent of the continental areas 
at the beginning of Paleozoic time was small, forming islands, 
Walcott is inclined to the belief that it was very considerable, 
stating : 
The continent was larger at the beginning of the Cambrian period than 
during any epoch of Paleozoic time,and probably not until the development 
of the great fresh-water lakes of the Lower Mesozoic was there such a broa 
1 Dr. Matthew writes me that he doubts if Hyalithoid shells should be referred 
to Pteropoda. “ Pelsineer quite repudiates them ; and to me their heavy shell, 
and frequent habitat on rough shores, do not speak of the fragile Pteropoda. 
