Fig. 35.—A specimen of Weymouthia nobilis (Ford), collected by Mr. Thomas H. Clark at North Weymouth, Mass. Note the broad smooth shields of this Lower Cambrian eodiscid. × 6.
In the discussion above I have placed great emphasis on the large size of the primitive pygidium, because, although there is nothing new in the idea, its significance seems to have been overlooked.
If the large pygidium is primitive, then multisegmentation in trilobites can not be primitive but is the result of adaptation to a crawling life. It is annelid-like, but is not in itself to be relied upon as showing relationship to the Chætopoda. Simple trilobites with few segments, like the Agnostidæ, Eodiscidæ etc., were, therefore, properly placed by Beecher at 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 Asaphidæ, it is learned that the geologically 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 genealogical 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 Eodiscidæ, and, in fact, one of the simplest trilobites known, for while it has three thoracic segments, it shows almost no trace of dorsal furrows 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 Kiær associated with Holmia and Kjerulfia at Tømten, Norway. It now appears it is really in its proper zone, and instead of being the most specialized, is the simplest of the Eodiscidæ.
What appears to be a still simpler trilobite is the form described by Walcott as Naraoia.
(Text [fig. 36].)
Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, p. 175, pl. 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 interpretation, 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, pl. 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.