286 AN INTRODUCTION TO THE STUDY OF FOSSILS 



ant. 



}■ cephaloTi 



thorax 



1 py^idi, 



urn 



B 



thora%-_;^^ 



ceph. 



vm. diqt U:" \ "■■,■:/" 



Q n\Q. "'• 



Fig. 126. — The trilobite, Triarthrus becki Green, from the Utica shales (mid-Ordo- 

 vician of eastern North America). This species flourished in the muddy portions 

 of the ocean which then covered this region. A, dorsal view. B, ventral view. 

 C, a transverse vertical section at second thoracic segment {A-C X i^). D, sl 

 median longitudinal vertical section. E and F ( X 45), two molts of the larval 

 (protaspis) stage ; the later molted one {F) has added to the pygidium an addi- 

 tional segment ; in neither are any thoracic segments yet developed ; the head has 

 five segments with the eye-lines extending from the first segment, a., anus; 

 ant., antennules or feelers ; ax., axial lobe ; ceph., cephalon or head segment ; 

 db., doublure; dig.t., digestive tube; e., eye lobe protecting the compound eye; 

 en., endopodite; ex., exopodite; Ja.s., facial suture; fi.ch., fixed cheeks; Jr.ch., 

 free cheeks; g/., glabella ; ^/a./., glabellar furrows; g«., gnathobase (the free, inward 

 projecting part of the protopodite) ; I. Jr., Hmb fringe's, for swimming and respira- 

 tion ; l.l., lower Up or metastome; mo., mouth; pg., pygidium, covering the ab- 

 domen ; pi., pleurae ; pr., protopodite ; u.L, upper lip or hypostome; v.m., ventral 

 membrane. (All after Beecher, except Z).) 



