some British Fossil Crustacea. 401 



by the facial suture ; eleven thoracic segments ; pleura simple, 

 obtuse ; pygidium as in Ceraurus. 



5. Acidaspis (Murch.) = Odontopleura (Em.). 



6. Staurocephalus (Bor.*). 



7. Remopleurides (Portk.t). 



8. ZethusJ (Pand., as defined by Volborth) = Cybele (Loven) 

 + Atractopyge (Hawle and Corda). 



3rd Subfam. Ogygin^:. 

 Body flat, broad oval ; thorax about as long as the head ; pleurae 

 flat, falcate, with a pleural groove not reaching the margin ; 

 ends not bent down, nor produced into spines ; no facets ; py- 

 gidium nearly as large as the head. 



This group would include (so far as I know) all flat-sided Tri- 

 lobites not entering into the Paradoxirue, but, unlike them, the 

 body is wide and short, the pygidium instead of being diminutive 

 is nearly as large as the head, and the segments are remarkably 

 few and never extend into spines. The eyes are small or absent. 



1. Trinucleus (Murch.). Head surrounded by a wide, pitted 

 margin ; six body-rings ; no eyes, cheeks not diagonally cut by 

 the eye-line. 



Subgen. 1. Tetrapsellium (H. & C). Only four body-rings. 



2. Tretaspis (M'Coy). Resembling Trinucleus, but the cheeks 

 divided by a diagonal eye-line, and with an ocular tubercle in 

 the middle ; five body-rings. (See description below.) 



3. Ampyx (Dal.). 



4. Ogygia (Brong.). 



Subgen. 1. Barrandia (M'Coy). (For characters, see below.) 



5. Bronteus (Gold.). 

 0. Lichas (Dal.). 



Subgen. 1. Trochurus (Bar.). 

 Subgen. 2. Acanthopyge (H. & C). 



4th Subfam. Harpedin^e. 

 Head large ; pygidium very small ; body long, rapidly tapering ; 

 pleurae abruptly bent down and obtuse at the ends ; no facets. 



* I have recently noted the S. Murchisoni in the Rhiwlas limestone. 



f I suspect the thoracic segments in this genus are only six to eight in 

 number, terminating at the long spines of the R. later ispinifer and R. dorso- 

 spinifer (Portk.), which I think probably mark the origins of the pygidium ; 

 but not having access now to perfect specimens of those rare Trilobites, I 

 can only offer these remarks as suggestions founded on analogy. 



X The genus Encrinurus seems closely allied in many respects to Zethus, 

 but differs by its simple, obtuse, thoracic segments ; not however being quite 

 sure of the structure of those latter, I am unwilling to assign the genus a 

 place in the system. 



Ann. $ Mag. N. Hist. Ser. 2. Vol. iv. 27 



