398 The American Geologist. June, 1897 
the natural arrangement, as, if we may judge by the analogy 
of the young of other trilobites, the generic features of the 
head-shield \yould have been developed, before the posterior 
segments of the body were defined. Such being the case we 
should look upon the characters of the head-shield as of vital 
importance in defining relationships of the trilobites. In this 
view it seems natural to look upon O. michwitzi as the oldest 
form known of the root stock of Olenellus, and to associate 
wMth it, as somewhat nearly related, O. cnllnvii and O. broggeri. 
In a discussion of the age of the Olenellus fauna it should 
be borne in mind that only 0.(C.) broggeri and Tlolmia kjeriiljf 
have been found in actual infra-position to Paradoxides. But 
there are several things which indicate the antiquity of 0. 
michwitzi, e. g., the long cylindrical glabella, the short pleura 
and the small size. On the other hand the dilferentiation of 
the thorax into two regions and the great thoracic spine, show 
that we are not dealing with the earliest representative of the 
stock.* 
The branch of the olenellid stem to which O. michwitzi be- 
longs, if we limit it to the three species above referred to, is 
pre-paradoxidean, and none of its species enter the Paradox- 
ides zone. But can we say the same of the stock at the base 
of which we find Ilolmia kjerulfi ? This branch of the ole- 
nellids is characterized by an enlarged front to the glabella 
after the manner of Paradoxides ; the feature is well marked 
in 0. {Elliptocephfila) asaphoiOes and is less distinctly shown 
in 0. [Ilesonacis) verm,ontmi<.i \ and although Mr. Walcott in- 
cludes the former species and the associated genera in his 
Olenellus fauna, he intimates that the}^ may be of the "Mid- 
dle" Cambrian, *. e., Paradoxides zone. 
In Jf. vei^morttana we seem to have the highest expression 
of this type of structure in a trilobite apparently adapted for 
living on a muddy bottom. With some primitive features, as 
for example, the long glabella, there are others which show an 
advanced stage of development, as the shortened eye-lobes, 
the numerous joints to the thorax and the dilferentiation of 
this part into two regions. This species, according to Wal- 
*0n account of these featvires, Peach thinks this species more modem 
than Holmiakjerulfl in which there is an equal development throughout; 
but for the reasons given above, and others, I do not concui- in this view. 
