TRILOBITA. 



353 



region of the stomach. The form of the glabella varies a 

 good deal. Usually it is widest in front (fig. 207), but its 

 width may be nearly uniform, or 

 it may be widest posteriorly and 

 contracted in front, as in Caly- 

 mene. The glabella is bounded 

 at the sides by two grooves, 

 which are known as the "axal 

 furrows," and is marked off be- 

 hind by a third groove, which 

 is termed the " neck - furrow." 

 The surface of the glabella may 

 be quite smooth, but it is ordi- 

 narily divided into " lobes " by 

 " grooves," which originate in the 

 axal furrows, and pass inwards 

 towards the middle line (fig. 207). 

 These furrows mark the position 

 of the segments which compose 

 the glabella, and they are some- 

 times 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 Illoenus (fig. 212), the glabella 

 is very indistinctly marked off from the rest of the shield. 



The grooves of the glabella probably mark off so many segments, to 

 which organs of prehension and mastication were attached inferiorly, and 

 they are marked internally by corresponding ridges, to which muscles 

 must have been attached. Sometimes (Illoenus, Ellipsocephalus, En- 

 crinurm, &c.) they are obsolete, as also occurs in particular species of 

 other genera (Trinucleus and jEglina). Typically, three pairs of grooves 

 are present, but Phacyps has four, and so have some other types. 



At each side of the glabella, and continuous with it, is a 

 small semicircular area, which is termed the " fixed cheek " 

 (fig. 206, /). The glabella, with the "fixed cheeks," is 

 separated from the lateral portions of the cephalic shield, 



VOL. I. Z 



Fig. 207. Phacops (Dal-manUrs) 

 limulurus. Upper Silurian. 



