THE NATURAL HISTORY SURVEY. 



179 



As has already been mentioned, the remaining genera of 

 trilobites present two distinct types of head structure dependent 

 upon the character of the free cheeks and the direction taken 



HYPOPARIA 





9 Agnostus 10 Microdiscus 



12 Trinucleus 



IS Ampyx 



FIGURE 14. Cephala of Hypoparia. 9, Agnostus; 10, Microdiscus; 

 u,Harpes; 12, Trinucleus; 13, Ampyx. (After Beecher.) 



by the posterior limbs of the facial sutures. In both these 

 types the free cheeks constitute an essential portion of the 

 dorsal surface of the head and are continued to the ventral 

 side only as a doublure or infolding of the margin. One type 

 of structure is characterized by having the free cheeks include 

 the genal angles, the distal extremity of the posterior limb 

 of the facial suture cutting the posterior margin of the head. 

 The genera possessing this type of structure constitute the 

 second order, the Opisthoparia. 



OPISTHOPARIA 



19 Illsnus 



20 Proetus 



21 Bronteus 



22 Lichas 



23 Acidaspis 



FIGURE 15. 14, Atops; 15, Conocoryphe; 16, Ptychoparia; 17, Olenus; 

 18, Asaphus; 19, Illacnus; 20, Proetus; 21, Bronteus; 22, Lichas; 23, Aci- 

 daspis. (After Beecher.) 



In those forms representing the third type of structure, 

 the genal angles are a part of the fixed cheeks, the posterior 

 limb of the facial sutures being directed laterally from the 

 posterior extremities of the eyes, their distal extremities cutting 

 the lateral margins of the head. The genera having this tpye 

 of structure are placed in the third order, Proparia. 



Among the Silurian trilobites described in this Bulletin, 

 -all three orders are represented. The Hypoparia, however, 



