610 



ARTHEOPODA 



SUB-KINGDOM VII 



the segments, the genal angles, and the pygidium. In rare instances, the 

 spines are solid. 



The axis, 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 

 posterior margin to the thorax. 

 The postero-lateral margins, or genal 

 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 

 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 fulcra 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 Aeglina, 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 Illaenus 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 

 cephalon. Sometimes the lateral furrows are continuous across the glabella, 

 or again, they may be directed obliquely (Triarlhrus), 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, 

 1265, B). 



Cephalon of Dalmanites Htiusmanni, 

 Brongt. sp. Devonian ; Bohemia. 

 I, Limb or border ; sin, Marginal 

 furrow ; a, Genal spines ; gl, Glabella ; 

 If, Frontal lobe; fl-P, 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. 



