222 
The American Geologist. October, moi. 
readily overturned and drifted up upon the beach, and the dif- 
ficulty of returning to their normal position increased by the 
flatness and width of their frontal carapace. 
Today the awkward and prolonged efforts of a Limulus 
to return to its normal position when upside down are finally 
rendered successful by the assistance of its telson. It can be 
pointed to as illustrating an effectual line of development 
which might have assisted the primordial trilobites, viz., flat 
and sloping heads, features absent in Dicellocephalus, Aglas- 
pis, Chariocephalus, Crepicephalus &c. 
Again it seems probable that the extended pleural segments 
of the primordial trilobites were correlated with a weak artic- 
ulation. This early crustacean failed in rigidity and coher- 
ence. It fell apart easily upon impact or collision or expos- 
ure. The line of favorable development in this case which 
would have strengthened the trilobite was in the direction of a 
shortened thoracic extent and an increased pygidium. But 
the surfaces of attachment for egg sacs were multiplied in the 
Cambrian multi-pleural forms and the intensity of multipli- 
cation was thereby augmented. 
From the early Cambrian Olenellus and the Paradoxides 
through Conocoryphe and Ptychoparia etc., to the Agnostus, 
Illaenus etc., there is an evident relative shrinkage in the ceph- 
alic shield, a condensation of the pleurse, a loss of pleural 
spines, a growth of the pygidium, all organic movements in the 
direction of strength and solidity. Environment and habit and 
selection played important parts in aiding a determined devel- 
opment, but the remarkable and overshadowing importance of 
crustacean life in the Cambrian day must have resulted from 
an optimum in the conditions then favorable to their growth, 
and from the ease also with which evolutionary processes 
formed them from an annelidan ancestor. 
This latter observation deserves a moment's clearer consid- 
eration. The development of a trilobite from an annelid would 
have been, humanly speaking, a more natural and hence a 
more quickly completed change than that of a brachiopod from 
the same original organism. If such evolutionary processes 
began at the same time, at a given period, subsequent to their 
inception, the trilobite phylum might have been numerously 
represented, wlien as yet the brachiopodous life had formed 
