14 



TRILOBITES. 



the component pleuree free at their extremities, simulating the character of neighbouring 

 genera, we cannot doubt that we are looking at a group of genera combined by common 

 characters into a natural family. 



If I express my own opinion, that the sub-generic groups here given are natural genera 

 in the ordinary sense of the word, and will be hereafter used as such — while for convenience 

 sake, and till the sub-divisions are completely established, we adopt the common term, it 

 will perhaps be all that in the present state of our knowledge is advisable. 



Genus — Phacops, Emmerich. Characters those of the familv. 



Range. Lowest Silurian — to Uppermost Devonian. 



Subgenus I. Trimerocephalus, M'Cog, 'Annals Nat. Hist./ 2nd series, vol. iv, 1S49- 



Fig. 5. 



P. (Trimeroc.) Volborthi, 

 Barr., Wenlock rocks of 

 Bohemia. 



Form compact, glabella inflated and expanded in front j the 

 lobes, except the basal ones, obscure. Eyes small, of few large 

 lenses (often ost by abrasion). Head-angles not spinous. 

 Pleura? all rounded. Tail small, of few segments, with even 

 border, and not at all produced. Ex. P. /avis, P. Volborthi, 

 P. micromma, &c. 



Mange. Upper Silurian — to Upper Devonian. 



Subgenus II. Phacops, Emmerich, 1845. 



Form compact, glabella inflated and expanded in front, the two front pairs of furrows 

 obscure. Eyes large and well developed, of numerous lenses. Head-angles not spinous. 

 Pleurae all rounded. Tail moderate, of few (often coalesced) segments, with an even 

 border, never produced. 



Range. Upper Silurian — to Uppermost Devonian. 



Subgenus III. Acaste, Goldfuss, 1845. 



Form compact, or at least not expanded. Glabella not inflated, nor much expanded 

 in front ; all the furrows distinct. Eyes well developed, of numerous lenses. Head-angles 



