AND DISTRIBUTION OF TRILOBITES. 263 
of which is directed exteriorly and anteriorly near the poste- 
rior margin. We have therefore to distinguish a threefold direc- 
tion of the facial line, starting from the posterior angle of the eye: 
—Ist, towards the external margin; 2ndly, towards the angle ; 
and, 3rdly, towards the posterior margin of the cephalic shield. 
Another characteristic furnished by the cephalic shield like- 
wise deserves attention, namely, the structure of the frontal part 
(Stirn-theil). It is only in few species that this part is uniformly 
arched, as in Nileus and Bumastus; generally speaking this also 
is tripartite. The two axal furrows of the body, which define the 
boundary between axis and fins, are continued over the cephalic 
shield ; in that case it is only in Jdlenus crassicauda that they 
end without meeting; however they generally meet and inclose 
the central part (which is usually arched) the so-called frontal 
protuberance or glabella. As each protuberance on the upper 
surface of the shell of the body, both in the shell of Trilobites 
as in that of recent Crustacea, corresponds with an indenta- 
tion on the lower side, and vice versd, the form of the glabella 
may not be without importance ; indeed it rather indicates es- 
sential differences of internal organization. Were we to find 
powerful jaws, we should be led to seek the places of application 
of the muscles moving them in some of these impressions, which 
are represented on the upper surface by protuberances ; but the 
accessory parts of the mouth, every vestige of which has disap- 
peared, scarcely required a highly developed muscular apparatus, 
and it is probable therefore that a portion of the more important 
intestines, the relative development of which caused the differ- 
ent vaulted appearances at various places on the cephalic shield, 
were situated beneath these elevations. But whatever may have 
produced the shape of the frontal part, it must be admitted to 
form an important characteristic. Its differences are of a twofold 
nature ; first, as to its dimensions; secondly, as to its articula- 
tion. It is either broadest towards the anterior part, that is 
clavate, which is most frequently the case; or it becomes nar- 
rower anteriorly, and has its greatest width at the neck, which 
is called conical or truncately conical ; in some few it is uniformly 
| broad everywhere ; in still fewer instances it is cylindrical and 
enlarged at the centre. It exhibits a second difference with re- 
gard to its more or less simple structure ; in some it is perfectly 
: simple (Homalonotus), and only intersected in the neck by the 
i 
posterior marginal or neck furrow; yenerally speaking it is di- 
