THE SIMPLEST TRILOBITE. 139 



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 Asaphidre, 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 P halacr oma bibullatutn 

 and P. niidutn 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 Eodiscidre, 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 Callaz'ia at North Weymouth, Massachusetts, and where it has lately been found 

 by Kircr 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 

 Eodiscid;t. 



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 

 antennas 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. 



