TRILOBITA. 



517 



exhibits more or less markedly a division into three longitudinal 

 lobes (fig. 368), from which the name of the order is derived. In 

 some cases, however, as in the genera Homalonotus and Illcenus, 

 this trilobation is only obscurely marked. Within the limits of the 

 same species, Barrande has observed that the individuals may some- 

 times present themselves under two forms, one broad and the other 

 long, and he regards the broad forms as the females, and the long 

 forms as the males, of the species. 



The crust exhibits a well-marked division into three regions, which 

 are commonly found detached and separate from one another. 

 These three regions are — 1, a cephalic shield; 2, a variable num- 

 ber of movable "body-rings " or thoracic segments ; and 3, a caudal 

 shield or " pygidium." 



The cephalic shield or buckler (fig. 368) is generally more or 

 less semicircular in shape, and is composed of a central and two 

 lateral pieces, of which the two latter may or may not be united in 

 front of the former. The central por- 

 tion of the cephalic shield is usually 

 elevated above the remainder. It is 

 termed the " glabella " (fig. 368,^), and 

 it protected the region of the stomach. 

 The form of the glabella varies a good 

 deal. Usually it is widest in front 

 (fig. 369), but its width may be nearly 

 uniform, or it may be widest pos- 

 teriorly and contracted in front, as 

 in Calymene. The glabella is bounded 

 at the sides by two grooves, which 

 are known as the " axal furrows," and 

 is marked off behind by a third groove, 

 which is termed the " neck-furrow." 

 The surface of the glabella may be 

 quite smooth, but it is ordinarily di- 

 vided into " lobes " by " grooves," 

 which originate in the axal furrows, 

 and pass inwards towards the middle 

 line (fig. 369). These furrows mark 

 the position of the segments which 

 compose the glabella, and they are 

 sometimes continuous from side to 

 side. Usually there are three pairs of these furrows, a lower or 

 basal, a middle or ocular, and an upper or frontal furrow ; but 

 there may be an additional pair of furrows in front of these. In 

 some cases, as in Illcenus (fig. 374), the glabella is very indistinctly 

 marked off from the rest of the shield. 



Fig. 369 



Phacops (Dalma?iites) 

 candata. Upper Silurian. 



