296 



CRUSTACEA. TRILOBITA 



Fig. 125. Calymene tuberculata, Silu- 

 rian. Ventral surface of head. 

 a, hypostome ; b, marginal rim ; 

 c, facial suture ; d, transverse su- 

 ture ; e, rostral plate. (After 

 Barrande.) Natural size. 



The head-shield is continued on the under surface of 

 the head as a reflexed 

 border or marginal rim 

 (fig. 125, b); sometimes 

 the facial sutures (c) are 

 continued across this bor- 

 der, and they may be 

 joined by a transverse 

 suture (d). Attached to 

 the border in the median 

 line is a plate (a), usually 

 oval or shield-shaped, 

 situated just in front of the mouth and known as the 

 hypostome or labrum (fig. 126). Just behind the mouth 

 is the small lower lip-plate or metastoma (fig. 128 A, m), 

 which, up to the present time, has been found in Tri- 

 arthrus only. 



In many Trilobites a small oval or elliptical area, 

 sometimes slightly raised like a 

 tubercle, in other cases depressed, 

 is found on each side of the hypo- 

 stome just behind the middle of 

 its outer surface (fig. 126); these 

 maculce are sometimes entirely 

 smooth, but in other cases a part, 

 or the whole of the surface, shows 



a structure similar to that of the compound eyes on the 

 dorsal surface of the head, and such were probably visual 

 organs. Macula? are not known to occur in any other 

 Crustacea. 



The thorax (fig. 123 B) consists of a series of segments, 

 which vary in number from two to twenty-nine, and are 

 movable upon one another, in some cases sufficiently to 



Fig. 126. Hypostome of 

 Asaphus tyrannus, from 

 the Llandeilo Beds. Re- 

 duced. 



