NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA SNODGRASS 83 



area of primary segmentation, for there is no doubt that the post- 

 cephaHc segments are produced by teloblastic growth. The youngest 

 trilobite larvae known give no evidence of metamerism (fig. 46 A), 

 but there soon appears in the glabellar region four pairs of lateral 

 impressions or transverse grooves that divide the glabella into five 

 consecutive lobes (fig. 36 A). These depressions produce internal 



Fig. 36. — Segmentation and tagmosis of Trilobita. 



A-D, four successive stages in larval development of Liostracus linnarssoni 

 Brogger (from Warburg, 1925). E, Olcnclhis vermoHtanus Hall (from Wal- 

 cott, 1910). F, Olenellus gilberti Meek (from Walcott, 1910). G, Schmidtiel- 

 lus mickwitsi Schmidt, distal body segments (from Walcott, 1910). H, 

 Asaphiscus wheelcri Meek, example of a trilobite with distal segments united 

 in a caudal fan, or pygidium (from Walcott, 1916). I, Agnostis montis Matthew, 

 example of the group Agnostia having only two free segments between head 

 and pygidium (from Walcott, 1908). 



jg, fixed cheek, or fixigene; jrl, frontal lobe; H, head; Ig, free cheek, or 

 libragene ; Pyg, pygidium ; sp, spine ; Tel, terminal lobe of body, probably the 

 telson ; Th, thorax ; ZG, zone of growth. 



ridges or apodemes most probably for muscle attachments, and their 

 formation, therefore, does not represent the process of segmentation 

 itself, but unquestionably they mark the primary intersegmental lines 

 of the segments united in the larval body. The first glabellar division, 

 known as the frontal lobe (A, frl), is continuous with a pair of 

 lateral areas {Ig) that become the "free cheeks" of the adult bearing 

 the compound eyes (fig. 46 E, Ig). The frontal lobe, therefore, may 



