286 AN INTRODUCTION TO THE STUDY OF FOSSILS 



"■ cephalon 





pyaidium 



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 13). -D, a 

 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-Hnes extending from the first segment, a., anus; 

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

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

 en., endopodite; ex., exopodite ; fa.s., facial suture; fi.ch., fixed cheeks; fr.ch., 

 free cheeks ; gl., glabella ; gla.f., glabellar furrows ; gn., gnathobase (the free, inward 

 projecting part of the protopodite) ; l.Jr., limb fringes, for swimming and respira- 

 tion ; /i., lower Hp or metastome; mo., mouth; />g., pygidium, covering the ab- 

 domen ; pL, pleura; ; pr., protopodite ; u.l., upper lip or hypostome ; v.m., ventral 

 membrane. (All after Beccher, except D) 



