EXTERNAL CHARACTERS 



223 



body is quite distinct in tlie majority of Trilobites, but in a few 

 genera belonging to tlie 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, 

 but varies considerably in different 

 genera. Only in a few cases, as in 



some S-pecieS of AgnOStuS (Fig. 146), Fig. IM.—HomaloHotus delphino- 

 . ., , ,, , ,1 -J. 1 1.1 cephalus. Green, x 1. Silurian. 



IS its length greater than its breadth. (After Zittei.) 

 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 (h). The shape of 

 the glabella varies greatly ; it may be oblong, circular, semi- 

 cylindrical, pyriform, spherical, etc. Its relative size likewise 

 varies ; thus in PhacoiJS cejjJiahtcs it expands in front and forms 

 the larger part of the head, whilst in Arcthusina (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, c, 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 



