NATURAL CLASSIFICATION OF THE TPdLOBITES 143 



ity of the Cambrian genera into families or the limitations 

 of the genera themselves must, as in the present instance, be 

 taken tentatively and as necessarily incomplete. 



A number of genera have been already made the types of 

 family divisions; as Faradoxides, Olenellus, Meniopleurides, 

 Ellipsocephalus, Ptyclioparia, etc. Some of them may be 

 shown ultimately to possess characters of sufficient weight 

 to be entitled to family distinction. A preliminary grouping 

 of the best-known genera may be of some value here, and 

 for the sake of convenience these divisions may be defined 

 as sub-families. Four groups will be recognized, of which 

 Paradoxides, Oryctocephalus, Olcnus, and Dikeloeephalus are 

 taken as representative genera. 



I. Paradoxinse. — Including Olenellus, Holmia, Mesonacis, 

 ^Iliptocephala, Schmidtia, Ohnelloides, Paradox'ides, Zacan- 

 thoides, and Meniopleurides. Most of the genera are distin- 

 guished by their long narrow eyes, often extending more 

 than half the length of the glabella, but more especially by 

 the rudimentary character of the pygidium. In Olenellus 

 the pygidium is a long telson-like sj^ine. In Holmia, Meso- 

 nacis, Elliptocephala, and Schmidtia it is reduced to a small 

 plate without distinct segmental divisions. In Paradoxides, 

 Zacanthoides, and Remopleurides the axis may show from one 

 to five annulations, while the limb may carry two or three 

 pairs of spines or may be entire. In Olenellus and Holmia 

 true facial sutures have been denied by some authors, but in 

 their place false sutures are recognized. They are, however, 

 evidently real sutures in a condition of symphysis, which 

 often occurs in Phacops, Proetus, Phillipsia, etc. Otherwise 

 these genera would violate the first f)rinciple of trilobite 

 structure, in not having the compound eyes on the free- 

 cheek pieces. Ohnelloides is a very striking form, but its 

 pygidium is unknown, and the head structure is obscure. 

 The elongate cephalon is a decidedly larval feature, and the 

 genal and intcrocular (?) spines strongly suggest its immature 

 condition, and point to the possibility of its being the young 

 of Olenellus or a related form. 



