610 ARTHROPODA SUB-KINGDOM VII 
the segments, the genal angles, and the pygidium. In rare instances, the 
spines are solid. 
The aais, or middle part, is defined by two longitudinal dorsal furrows 
extending the whole length of the thorax, and also over more or less of the 
cephalon and pygidium. 
The pleura are the two lateral areas on each side of the axis. Thus, there 
are pleural cephalic, thoracic, and pygidial regions. The name pleuron or 
pleura is especially applied to the extensions from the axial portion of each 
free segment. 
The Cephalon.—The cephalon, or cephalic shield (Fig. 1261) includes all 
that part of the carapace in front of the thorax. It comprises the hypostoma, 
epistoma, the free cheeks bearing 
the eyes, the fixed cheeks, and the 
glabella ; it is generally semicircular 
in form, and is joined along its 
oc _. Gat © Zee \ posterior margin to the thorax. 
! | if The postero-lateral margins, or genal 
S// \ : AS angles, are frequently drawn out 
into spines. Usually there is an 
occipital furrow extending across 
the cephalon parallel to the posterior 
margin, and defining the occipital 
ring or segment. 
The glabella is the axial portion 
of the cephalon, and is defined by 
the primary dorsal furrows (Fig. 
1261). It shows typically three 






Fic. 1261. 
Cephalon of Dalmanites Huusmanni, 
Brongt. sp. Devonian; Bohemia. 
1, Limb or border; sm, Marginal 
furrow ; a, Genal spines ; gl, Glabella ; 
lf, Frontal lobe; /1-18, Side lobes ; 
1-3, Side furrows of glabella; so, Neck 
furrow; a, Neck ring; ©, Facial 
suture ; oc, Visual surface of the eyes ; 
p, Palpebral lobe. oblique or transverse furrows in 
addition to the occipital ring, mark- 
ing the limits of the original five consolidated segments, and corresponding to 
the paired appendages of the ventral side. Sometimes the positions of the 
muscular fulera are also indicated on the dorsal surface, by short furrows, or 
by shallow pits. The glabella may constitute nearly the whole of the cephalon, 
as in Deiphon or Aecglina, or it may be narrow, as in Harpes, and Eurycare. | In 
some cases it does not extend over half the length of the cephalon, as in Harpes 
and Arethusina, but it may extend to the frontal border, as in Placoparia or 
Calymmene, or even beyond, as in Phacops, Ampyx, and Conolichas. The entire 
portion of the glabella which lies in front of the anterior lateral furrows, and 
which is often somewhat enlarged laterally, is called the frontal lobe. At 
times the limitation between the glabella and fixed cheeks is scarcely defined, 
as in L/laenus and Dipleura. Most frequently, however, three pairs of grooves 
can be distinguished in front of the neck furrow, marking the pentamerous 
division of the glabella and the five pairs of appendages attached to the 
eephalon. Sometimes the lateral furrows are continuous across the glabella, 
or again, they may be directed obliquely (Z’riarthrus), or even form longi- 
tudinal grooves (Conolichas). 
The hypostoma, or labrum, is homologous to the upper lip of other 
Crustaceans, and consists of a separate plate attached by an articulating 
surface or line to the reflexed border of the cephalic shield (Figs. 1262, 
5, B). 
