138 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRII.OBITES. 
due to imperfect preservation of the exceedingly small shells, which practically always occur' 
as moulds or casts in soft shale. There is, however, a very general increase in the strength 
of glabellar segmentation in the early part of the ontogeny of all trilobites whose life his- 
tory is known, and in some genera, like the Agnostidae, there is no question of the compara- 
tively late acquisition of glabellar furrows. Even in Parad oxides, the furrows appear late 
in the ontogeny. 
Summary. 
If absence of eyes on the dorsal surface be primitive, as Beecher has shown, and if 
the large pygidium, narrow axial lobe, and long unsegmented glabella be primitive, then 
the known protaspis of the Mesonacidae and Paradoxidae is not primitive, that of the Olen- 
idae is very primitive, and that of the Agnostidae is primitive except that in one group the 
axial lobe, when it appears, is rather wide, and in the other a brim is present. 
Fig. 35- — A specimen of Wey- 
mouthia nobilis (Ford), col- 
lected by Mr. Thomas H. Clark 
at North Weymouth, Mass. 
Note the broad smooth shields 
of this Lower Cambrian eodis- 
cid. X 6. 
Subsequent development from the simple unsegmented protaspis would appear to show, 
first, an adaptation to swimming by the use of the pygidium; next, the invagination of the 
appendifers as shown in the segmentation of the axial lobe indicates the functioning of the 
appendages as swimming legs; then with the introduction of thoracic segments the assump- 
tion of a bottom-crawling habit is indicated. Some trilobites were fully adapted for bottom 
life, and the pygidium became reduced to a mere vestige in the production of a worm-like 
body. Other trilobites retained their swimming habits, coupled with the crawling mode of 
life, and kept or even increased (Isotelus) the large pygidium. 
The Simplest Trilobite. 
In the discussion above I have placed great emphasis on the large size of the primi- 
tive 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 primi- 
tive 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 Chastopoda. Simple trilobites with few seg- 
ments, like the Agnostida;, Eodiscida;, etc., were, therefore, properly placed by Beecher at 
