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, Griffithides, &e. 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 rigs 
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 pleure 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- 
lheve to be of only specific importance ; and the former, though 
of imperfect application as stated by the author, becomes m- 
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, flat, trian- 
gular space at the extremity of the anterior margin of the pleurse 
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 pleure ;—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 pleure 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 mdicate the number of thoracic 
segments in each genus. 
I propose dividing the family of Trilobites into the five follow- 
ing subfamilies :—1. Asaphine; 2. Paradoxine ; 3. Ogygine ; 
* The term ‘fulcrum,’ as sometimes applied to a point on the anterior 
edge of the pleurz, clearly conveys a false mechanical notion, besides being 
synonymous with the already current term ‘knee’ used by Pander and 
Portlock. 
’ 
