TRILOBITES. 



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spinous. Pleurae rounded or truncate, not produced into spines. Tail moderate, of less 

 than eleven segments, with an even border, but often mucronate. 

 Range. Lower and Upper Silurian. 



Subgenus IV. Chasmops, McCoy, 1849. 



Form rather large and depressed. Glabella greatly expanded in front, the lobes 

 unequal, the hinder ones being contracted, and almost obsolete ; the front pair greatly 

 expanded, and overlapping the others. Head-angles spinous. Pleurae truncate. Tail 

 large, of few or many segments, not dentate, and seldom mucronate. 



Subgenus V. Odontochile, Corda, 1847. (Dalmania, Emmerich, 1845.) 



Form large, depressed. Glabella depressed, not much expanded in front; all the 

 lobes distinct, the front ones not enlarged at the expense of the others. Head-angles 

 long-spined. Pleura? truncate, and the hinder ones often produced. Tail .large, of more 

 than eleven segments, with an even border, — often mucronate. 



Range. — Lower Silurian, rare; Upper Silurian, common. 



Subgenus VI Cryphceus, Green, 1837. 



Form of moderate size, depressed. Glabella depressed, not much expanded in front. ; 

 all the lobes distinct, the front ones not greatly enlarged. Head-angles long-spined. 

 Pleurae truncate, and the hinder ones often produced, into spines. Tail large, of many 

 segments ; the margin spinose. 



I believe there are more subgenera of Phacops to be discovered and described yet. For 

 instance, there is probably more than one included here under Trimeroceplialus, and there 

 is a South African form of Cryphceus, which has the characters of Acaste in the compact 

 habit and convex tail. All the other subgenera also contain species which it would be 

 difficult to assign rightly to one or the other. 



It will be observed that we have here a wide range of characters, and in regular 

 gradation, from the most expanded, flattened forms, with glabella deeply lobed, and the 

 segments of the large tail almost free, to the most compact and rounded form, with the 

 caudal extremity reduced in size, of even contour, and with the segments most coalesced. 

 We begin with the subgenus Trimeroceplialus. 



