THE TRILOBITA AND OTHER ARTHROPODA 207 



ORDER I. : OPISTHOPARIA. 



Facial suture cutting posterior margin. 



SUB-ORDER i. Mesonacida. Long crescentic eyes 

 close to glabella, wide free cheeks (in earliest genera, 

 facial suture not yet developed). Nearly all in micro- 

 pygous stage ; thoracic pleurae to a large extent free 

 from one another, the free part (pleural spines) more and 

 more curved back towards posterior end. Mainly 

 Lower and Middle Cambrian, including the zonal genera 

 Olenellus (Figs. 59, a; 61, a) and Paradoxides ; the special- 

 ized pelagic Remopleurides is Ordovician. 



SUB-ORDER 2. Conocoryphida. Glabella' usually 

 parallel - sided or narrowing forwards, eyes small ; 

 thoracic pleurae not spiny ; pygidium in most cases 

 hetero- or iso-pygous. A very large group, ranging 

 through the whole Palaeozoic era, and subdivisible into 

 sections : 



(i.) Conocoryphina : specialized in respect of blindness, 

 but otherwise the most primitive ; glabella with oblique 

 segmental furrows ; free checks extremely narrow ; 

 micro- to hetero-pygous. Cambrian. 



(ii.) Olenina, including two main families, the earlier 

 Oknida with broad short head, narrow glabella, tapering 

 thorax of twelve to twenty-two somites, micro- to hetero- 

 pygous, Cambrian (Fig. 59, c) ; and the later Proetida with 

 longer head, thorax of eight to ten somites, not tapering, 

 hetero- to iso-pygous (Ord.-Perm.) the only family 

 surviving the Devonian (Fig. 59, g). 



(iii.) Ptychoparina, including Cambrian heteropygous 

 families near to Conocoryphidae but with eyes, and two 

 later isopygous families Asaphida (Figs. 59, b; 6i,b, c\ 

 with eight thoracic somites (Camb.-Ord.), and Illanida 

 (Fig. 59, d, e) with ten (Ord.-Sil.). 



(iv.) Calymenina, a small group in which the facial 



