354 ARTHROPODA. 



termed the " movable " or " free cheeks/' by a peculiar suture 

 or line of division, which is known as the " facial suture " 

 (fig. 206, /). No such peculiar line of division is known to 

 exist in any recent Crustacean ; but there is a faint indi- 

 cation 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 (Asa- 

 phus, Phacops, Homalonotus, &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, Micro- 

 discus, and Agnostus) the facial suture is 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 corn- 

 pound, and consist of an aggregation of facets, covered by a 

 thin cornea. They are generally crescentic or reniform in 

 shape, and are invariably sessile, in the sense that they are 

 never supported upon movable stalks. In some cases, how- 

 ever, they are carried upon longer or shorter prominences. 

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

 forms, such as Agnostics, Ampyx, some of the Trinuclei, and 

 certain forms of Conocephalus. Though usually facets are 

 easily detected in such as have eyes of any size, there are 

 some (Brontem, Arethusina, Proetus, &c.) in which the eyes 

 are smooth. In any case, the number of lenses varies greatly, 

 there being as few as fourteen facets, or as many as fifteen 

 thousand in each eye in different types (Barrande). 



Behind the cephalic shield comes the thorax, composed 



