240 TRILOBITA CHAP. 
to reach the anterior margin. In this species the eye-line is 
present in the adult. 
In the protaspis of Zriarthrus (Fig. 144), found in the 
Ordovician, the glabella does not reach the front margin nor 
expand in front as it does in 
Sao; an eye-line is present, 
but disappears before the adult 
stage is reached. 
Dalmanites (Fig. 151, C) 
is a more advanced type than 
Sao and Triarthrus, and is 
found in later deposits. In 
the earliest stage (Fig. 145, A) 
Fic. 144.—Triarthrus becki, Green. Ordo- the head and pygidium are 
vc, B Two acconive tages of the suite distinet, and there is 
no eye-line present at this or 
any stage in development, but large ovoid eyes are found on 
the front mafgin, and have their long axes placed transversely 
Grane 
Fig. 145.—Larval stages of Trilobites. A-D, Dalmanites socialis, Barr. Ordovician, 
Bohemia. The small figures below show the natural size of each specimen. (After 
Barrande.) E, Mesonacis asaphoides, Emmons, x 10. Lower Cambrian, North 
America. (After Walcott.) F, Acidaspis tuberculata, Conrad, x 20. Lower Helder- 
berg Group (Lower Devonian or Upper Silurian), Albany County. (After Beecher.) 
to the axis of the body; the glabella is strongly segmented and 
is rounded in front. In later stages (C, D) the pygidium 
increases in size relatively, and the thoracic segments are 
successively introduced ; the facial sutures and free cheeks appear 
