CRUSTACEA. TRILOBITA 



301 



margin of the head. In later stages the pygidium becomes 

 more distinct and increases in size ; and the thoracic seg- 

 ments are gradually introduced between the head and the 

 pygidium (C, D). At the same time the eyes move back- 

 wards and inwards until they attain their adult position, 

 and the free cheeks increase in size (H). The glabella 



Fig. 129. Development of Trilobites. (After Barrande.) 



A — D. Sao hirsuta, Cambrian, Bohemia. A, earliest stage (protaspis), 



x 12. B, later stage with three segments behind the head, x 12. 



C, with more distinct glabella furrows, and four segments behind 



the head, x 12. D, with six segments behind the head, x 10. 



E — H. Phacops (Odontochile) socialis, Ordovician, Bohemia, x about 8. 



E, earliest known stage, with eyes at the margin, and three segments 



behind the head. F, later stage with more distinct furrows on the 



glabella, and four segments behind the head. G, with eyes moved 



inward, and narrow free cheeks ; six segments behind the head. 



H, free cheeks relatively larger and eight segments behind the head. 



often becomes rounded in front and relatively shorter ; its 

 furrows become more distinct, indicating the existence of 

 five cephalic segments. Since in the earliest stages of all 

 Trilobites the free cheeks are either absent from the dorsal 

 surface or are narrow, Beecher regards the Trilobites 

 which retain this character in the adult state as more 

 primitive than those in which the free cheek becomes 



