some British Fossil Crustacea. 401 
by the facial suture ; eleven thoracic segments ; pleure simple, 
obtuse ; pygidium as in Ceraurus. 
. Actpasprs (Murch.) = Odontopleura (Em.). 
. STAUROCEPHALUS (Bor.*). 
. Remopievripes (Portk.+). 
. Zeruust (Pand., as defined by Volborth) = Cybele (Loven) 
+ Atractopyge (Hawle and Corda). 
on Dd o 
3rd Subfam, Ocyeina. 
Body flat, broad oval ; thorax about as long as the head ; pleure 
flat, Plena, with a ‘pleural g eroove not reaching the rene gin ; 
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 Paradoxine, but, unlike them, the 
body is wide and short, the pygidium instead of bemg diminutive 
1s nearly as large as the head, and the segments are : remarkably 
few and never extend into spines. The eyes are small or absent. 
1. Trrnuctevs (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. Treraspis (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. Oayera (Brong.). 
Subgen. 1. Barrandia (M‘Coy). (For characters, see below.) 
5. Bronreus (Gold.). 
6. Licuas (Dal.). 
Subgen. 1. Trochurus (Bar.). 
Subgen. 2. Acanthopyge (H. & C.). 
4th Subfam. Harpepin&. 
Head large ; pygidium very small; body long, rapidly tapering ; 
pleurze abruptly bent down and obtuse at the ends ; no facets. 
* I have recently noted the S. Murchisoni in the Rhiwlas limestone. 
+ I suspect the thoracic segments in this genus are only six to eight in 
number, terminating at the long spines of the 2. laterispinifer and R. dorso- 
spinifer (Portk.), which [ 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. 
{ 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 
