SUBCLASS I 



TRILOBITA 



695 



by minute pores, which give a punctate appearance to the test, and which are 

 sometimes large, as in Homalonotus and related forms. 



The carapace is somewhat arched or convex, generally elongate-oval in 

 form, and usually rounded at both ends. The length is almost invariably 

 greater than the width. Very often the same species shows a broad form, 

 as well as a relatively larger, narrower one. The former was considered i by 

 Barrande as representing the female, and the latter the male individual. The 

 carapace is often ornamented with spines, teeth and knobs. These may be of 

 the nature of surface ornaments, or in the case of spines, may be produced by 

 growths from the genal angles, the ends of the segments of the thorax and 

 pygidium, or the spiniform extension of the pygidial termination. 



The carapace does not often terminate at the margin as a simple lamellar 

 plate, but is turned under, and forms a reflexed margin, or doublure, which is 

 parallel to the outer edge, but is separated from the upper surface by a narrow, 

 partially included space. This produces the hollow spines from the ends of 

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

 the spines are solid. 



The axial lobe, 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 ])leura are the two lateral areas on each side of the axis. Thus, there 

 are pleural cephalic, thoracic and pygidial regions. The name pleuron (in 

 the singular), or pleura, is especially applied to the extensions from the axial 

 portion of each free segment. 



The Cephalon. — The cephalon, or cephalic shield (Fig. 1337), 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. 

 1337). 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 Dciphon or Aeglina, or it may be narrow, as in Harpes and Enrycare. In 

 some cases it does not extend over half the length of the cephalon, as in Harpes 



It, Genal spines ; gl, Glabella ; If, 

 Frontal lobe ; 1^43, Side lobes ; 1-3, 

 Side furrows of glabella ; so, Neck 

 furrow ; A, Neck ring ; (S, Facial 

 suture ; oc. Visual surface of the eyes ; 

 p, Palpebral lobe. 



