some British Fossil Crustacea. 397 



front of the glabella, in the furrow which surrounds it, and in 

 which, when clear of matrix, I have observed them in Trinucleus, 

 Acidaspis, Calymene, Ampyx, Griffit hides, &c. We would thus 

 have the cephalic shield of Trilobites composed of an extension 

 of the two first cephalic rings. The protuberance called the glabella 

 probably contains the stomach, which is always in Crustacea large 

 and over the mouth ; the segmental furrows indicating the rings 

 which bear the parts of the mouth. 



After much labour in investigating the characters of Trilobites, 

 I venture to propose the following classification of the group, 

 founded in the first instance on a consideration of the variations 

 in structure of the pleurce or lateral portions of the thoracic seg- 

 ments, which I find to afford definite characters, easily found in 

 all moderately well-preserved specimens. The two principal me- 

 thods hitherto proposed fall far short of a natural or satisfactory 

 classification ; — that of Dr. Burmeister taking as the principle of 

 division, the presence or absence of the power of rolling into a ball ; 

 and Hawle and Corda resting their great divisions on the integrity 

 or denticulation of the edge of the pygidium. The latter I be- 

 lieve to be of only specific importance ; and the former, though 

 of imperfect application as stated by the author, becomes in- 

 cluded in the following arrangement. An extended examination 

 of the subject will show that Quenstedt, &c. cannot be followed in 

 the attempt to base the primary divisions on the number of the 

 thoracic segments — I have satisfied myself at least that that cha- 

 racter loses among the Entomostraca the importance which it 

 bears among the other Crustaceans, and that in the present 

 family it is only of subgeneric value. In the following remarks 

 I introduce two new terms — "facet " for the smooth, fiat, trian- 

 gular space at the extremity of the anterior margin of the pleurae 

 of certain Trilobites — and " pleural groove" for the shallow sulcus 

 which extends from the axis a variable distance towards the ex- 

 tremity of each of the pleurse ; — it is to the under side of this lat- 

 ter, as suggested by Burmeister, that the gill-feet were probably 

 attached*. To facilitate the appreciation of those characters, I 

 subjoin sketches of the pleurse of the more important genera, as 

 the needful information is not given in the greater number of 

 figures and descriptions of Trilobites hitherto published ; the nu- 

 merals prefixed to each figure indicate the number of thoracic 

 segments in each genus. 



I propose dividing the family of Trilobites into the five follow- 

 ing subfamilies : — 1. Asaphince; 2. Paradoxince ; 3. Ogygince ; 



* The term 'fulcrum,' as sometimes applied to a point on the anterior 

 edge of the pleurse, clearly conveys a false mechanical notion, besides being 

 synonymous with the already current term 'knee' used by Pander and 

 Portlock. 



