5 1 8 CRUSTACEA. 



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 {Illcemts, Ellipsocephahcs, Ehcrin- 

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

 genera {Trinucleus and ALglina). Typically, three pairs of grooves are 

 present, but some species of Phacops have four, and so have other types. 



In certain Trilobites there are found two pores, or, in other cases, 

 funnel-shaped depressions, one on each side of the glabella, in the axal 

 furrow which separates this region from the cheeks. M'Coy thought 

 that these " cephalic pores " might have been the points of origin of a 

 pair of antennae. It is certain, however, that this cannot be the true 

 explanation of these structures ; and they may be perhaps accounted for 

 as marking the point of origin of deep internal processes of the exo- 

 skeleton to which muscles were attached, while it has also been suggested 

 that they may indicate the position of a pair of " ocelli." 



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

 semicircular area, which is termed the "fixed cheek" (fig. 368,^). 

 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. 368, /). No such line of division 

 is known to exist in any recent Crustacean ; but there is a faint 

 indication of it in Limulus, 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 {Asaphus, Phacops, Homa- 

 lonotus, &c.) 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 — 1, the glabella and fixed cheeks ; and 2, the amalgamated 

 free cheeks. In other cases (Paradoxides, Illcenus, Proetus, &c), 

 the facial sutures, instead of joining in front of the glabella, are 

 continued forward, till they cut the anterior margin 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 

 Trinucleus, Microdiscus, and Agnostus) the facial sutures are absent, 

 in which case the free and fixed cheeks are fused with one another. 



The posterior angles (" genal angles ") of the free cheeks are very 

 commonly prolonged into longer or shorter spines, and the free 

 cheeks also bear the eyes. The eyes are compound, and consist of 

 an aggregation of facets, covered by a thin cornea. They are gen- 

 erally crescentic or reniform in shape, and are invariably sessile, in 

 the sense that they are never supported upon movable stalks. In 

 some cases, however, they are carried upon longer or shorter prom- 

 inences. The eyes differ much in size, and they are wanting in a 



