240 



TRILOBITA 



to reach the anterior margin. In this species the eye-line is 



present in the adult. 



In the protaspis of Triarthrus (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) 



FIG. 144. Triarthrus becki, Green. Ordo- the head and pygidium are 

 vician. A, B, Two successive stages of the m .:. p rHsHnnr inrl thprp is 

 protaspis, x 45. (After Beecher.) <l m Ct > ' 



no eye-line present at this or 



any stage in development, but large ovoid eyes are found on 

 the front margin, and have their long axes placed transversely 



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 tuberciilata, 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 



