NATURAL CLASSIFICATION OF THE TRILOBITES 185 



free-cheeks in other trilobites, especially where they are con- 

 tinuous around the front of the cephalon, as in Trinucleus 

 and Ampyx. The presence of a hypostoma in Agriostus was 

 also determined. Even in the higher genera of this order 

 the suture is frequently unnoticed in descriptions, but it can 

 be seen in all well-preserved specimens. In Trinucleus ^^ and 

 Harpes it follows the edge of the cephalon, and separates 

 the dorsal from the ventral plate of the pitted limb. Since 

 eye-spots occur on the fixed-cheeks in the young Trinucleus 

 and adult Harpes^ it is probable that this character is a 

 primitive one in this order, and has been lost in Aynostus^ 

 Microdiscus^ Ampyx, and Dionide. 



The ontogeny of Sao, Ptyclioparia, Triarthrus, Dalmanites, 

 etc., shows that the true eyes and free-cheeks are first devel- 

 oped ventrally, appearing later at the margin, and then on 

 the dorsal side of the cephalon. Therefore the Agnostidse, 

 Trinucleida?, and Harpedidse have a very primitive head struc- 

 ture, characteristic of the early larval forms of higher families. 

 Other secondary features show that this order, though the 

 most primitive in many respects, is more specialized than 

 either of the others, except in their highest genera. The 

 characters referred to are the glabella and pygidium. Very 

 few species show the primitive segmentation of the gla})ella, 

 it being usually smooth and inflated, and resembling in its 

 specialization such higher genera as Proetus, Asaphus, and 

 Lichas. The pygidium often fails to indicate its true num- 

 ber of segments. Some Aynostus and Microdiscus show no 

 segments either on the axis or limb of the pygidium. Trinu- 

 cleus and others may have a many-annulated axis and fewer 

 grooves on the pleural portions. The number of appendages 

 corresponds to the axial divisions, as determined by the 

 writer.'* Tlie multiplication of segments in the pygidium and 

 their consequent crowding makes them quite rudimentary. 



Family I. Agnostid^ Dalman. 



Small forms, having the cephalon and pygidium elongate, 

 nearly equal, and similar in form and markings. Free-cheeks 



