138 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES. 



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 Agnostid;e, there is no question of the compara- 

 tively late acquisition of glabellar furrows. Even in Paradoxidcs, 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 Paradoxidre is not primitive, that of the Olen- 

 idse is very primitive, and that of the Agnostidre 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- 

 id. 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 Quctopoda. Simple trilobites with few seg- 

 ments, like the Agnostidse, Eodiscidae, etc., were, therefore, properly placed by Beecher at 



