VII EXTERNAL CHARACTERS 2 
to 
On 
body is quite distinct in the majority of Trilobites, but in a few 
genera belonging to the Asaphidae and Calymenidae (Fig. 136) 
it becomes more or less completely 
obsolete. 
In most cases the only part of 
the Trilobite which is preserved is 
the exoskeleton which covered the 
dorsal surface of the body. That 
skeleton consists largely of calcare- 
ous material, and shows in sections a 
finely perforated structure. Gener- 
ally it is arched above, but in some 
cases is only slightly convex; in 
outline it is more or less oval. 
Three regions can always be dis- 
tinguished in the body of a Trilo- 
bite—the head, the thorax, and the 
abdomen or pygidium. 
The carapace which covers the 
head is known as the cephalic shield 
(Fig. 137, A, 1), and is commonly | : 
more or less semicircular in outline, (j= 
but varies considerably in different re 
genera. Only in a few cases, as in 
i 
some species of Agnostus (Fig. 146), Fic. 136.—Homalonotus delphino- 
es é cephalus, Green, x 1. Silurian. 
is its length greater than its breadth. (After zittel.) : 
The axial part of the cephalic shield, 
called the “glabella” (Fig. 137, A, a), is usually more convex 
than the lateral parts (“cheeks” or “genae”), and is separated 
from them by longitudinal or axial furrows (2). The shape of 
the glabella varies greatly; it may be oblong, circular, sem1- 
cylindrical, pyriform, spherical, etc. Its relative size likewise 
varies; thus in Phacops cephalotes it expands in front and forms 
the larger part of the head, whilst in Arethusine (Fig. 151, B) 
it is narrow and short, being only about one-half of the length 
of the head. 
The segmentation of the head is indicated by transverse 
furrows on the glabella (Fig. 137, A, ¢, d). In some. cases 
these furrows extend quite across the glabella (Fig. 147), but 
commonly they are found on the sides only and divide the 
