162 



ANNULOSA. 



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 Illanus (fig. 109), the gla- 

 bella is very indistinctly marked 

 off from the rest of the shield. 



At each side of the glabella, 

 and continuous with it, is a small 

 semicircular area, which is 

 termed the " fixed cheek " (fig. 

 103, 2). The glabella, with the 

 " fixed cheeks," is separated 

 from the lateral portions of the 

 cephalic shield, termed the 

 "movable" or "free cheeks," 

 by a peculiar suture or line of 

 division, which is known as the 

 " facial suture" (fig. 103, 2, e t). 

 No such peculiar line of division 

 is known to exist in any recent 

 Crustacean ; but there is a faint 

 indication of it in Limn/us, and 

 some doubtful traces of it in certain other forms. The course 

 taken by the facial sutures differs in different cases, and 

 causes an important difference in the structure of the cephalic 

 shield. In some cases, the facial sutures, starting from the 

 posterior margins of the buckler, skirt the fixed cheeks, and 

 join one another in front of the glabella. In these cases 

 it is obvious that the free cheeks form a single piece, so that 

 the entire shield consists of but two portions i, the glabella 

 and fixed cheeks ; and 2, the amalgamated free cheeks. In 

 other cases, the facial sutures, instead of joining in front of the 

 glabella, are continued fonvard, till they cut the anterior mar- 

 gin of the shield separately. In these cases the free cheeks 

 are discontinuous, and the cephalic shield consists of three 

 portions. In a few genera (as in Trinuc/cus, Microdiscus, and 

 Agnostus), the facial suture is absent. 



The posterior angles of the free cheeks are very commonly 

 prolonged into longer or shorter spines, and they bear the 

 eyes. The eyes are compound, and consist of an aggregation 



Fig. io4-PAacofs (Dalmanites) 

 limulurus, Upper Silurian. 



