THE SIMPLEST TRILOBITE, 1 39 
the base of his classification, and there is now less chance than ever that they can be called 
degenerate animals. 
From the phylogeny of certain groups, such as the Asaphida?, it is learned that the geo- 
logically older members of the family have more strongly segmented anterior and posterior 
shields than the later ones. That there has been a "smoothing out" is demonstrated by 
a study of the ontogeny of the later forms. From such examples it has come to be thought 
that all smooth trilobites are specialized and occupy a terminal position in their genealogi- 
cal line. This has caused some wonder that smooth agnostids like Phalacroma bibullatum 
and P. nudum should be found in strata so old as the Middle Cambrian, and was a source 
of great perplexity to me in the case of Weymouthia (Ottawa Nat., vol. 27, 1913) (fig. 
35). This is a smooth member of the Eodiscida;, and, in fact, one of the simplest trilo- 
bites known, for while it has three thoracic segments, it shows almost no trace of dorsal fur- 
rows or segmentation on cephalon or pygidium, and, of course, no eyes. Following the 
general rule, I took this to be a smooth-out eodiscid, and was surprised that it should come 
from the Lower Cambrian, where it is associated with Elliptocephala at Troy, New York, 
and with Callavia at North Weymouth, Massachusetts, and where it has lately been found 
by Kiffir associated with Holmia and Kjerulfia at T<£mten, Norway. It now appears it is 
really in its proper zone, and instead of being the most specialized, is the simplest of the 
Eodiscidre. 
What appears to be a still simpler trilobite is the form described by Walcott as Naraoia. 
Naraoia compacta Walcott. 
(Text fig. 36.) 
Illustrated : Walcott, Smithson. Misc. Coll., vol. 57, 1912, p. 175, pi. 28, figs. 3, 4. — Cleland, Geology, 
Physical and Historical, New York, 1916, p. 412, fig. 382 F (somewhat restored). 
This very imperfectly known form is referred by Walcott to the Notostraca on 
what appear to be wholly inadequate grounds, and while I do not insist on my interpreta- 
tion, I can not refrain from calling attention to the fact that it can be explained as the 
most primitive of all trilobites. It consists of two subequal shields, the anterior of which 
shows slight, and the posterior considerable evidence of segmentation. It has no eyes, no 
glabella, and no thorax, and is directly comparable to a very young Phalacroma bibullatum 
(see Barrande 1852, pi. 49, figs, a, b). Walcott states that there is nothing to show how 
many segments there are in the cephalic shield, but that on one specimen fourteen were 
faintly indicated on the abdominal covering. The appendages are imperfectly unknown, as 
no specimen showing the ventral side has yet been described. The possible presence of 
antenna; and three other appendages belonging to the cephalic shield is mentioned, and there 
are tips of fourteen legs projecting from beneath the side of one specimen. As figured, 
some of the appendages have the form of exopodites, others of endopodites, indicating that 
they were biramous. 
Naraoia is, so far as now known, possessed of no characteristics which would prevent 
its reference to the Trilobita, while the presence of a large abdominal as well as a cephalic 
shield would make it difficult to place in even so highly variable a group as the Branchi- 
opoda. On the other hand, its only exceptional feature as a trilobite is the lack of thorax, 
and all study of the ontogeny of the group has led us to expect just that sort of a trilo- 
bite to be found some day in the most ancient fossiliferous rocks. Naraoia can, I think. 
