CRUSTACEA. 



161 



any rate, have been flexible and membranous; since many 

 species have the power of rolling themselves up into a ball like 

 the recent wood-lice (Oniscus). Other Trilobites, however, 

 never seem to have possessed any power of rolling up. The 

 dorsal crust usually exhibits more or less markedly a division 

 into three longitudinal lobes (fig. 103), from which the name 



Fig. 103. Morphology of Trilobites. i. Angelina Sedgwickii, Upper Cambrian. 

 2. Diagram of a Trilobite (after Salter) : a Glabella with its furrows; bb Free cheeks, 

 bearing the eyes (po) ; c c Fixed cheek, including the eye-lobe (d)', ee Facial suture. 



of the order is derived. In some cases, however, as in the 

 genera Homalonotus and Illanus, this trilobation is only ob- 

 scurely marked. The crust exhibits a well-marked division 

 into three regions, which are commonly found detached and 

 separate from one another. These three regions are i, a 

 cephalic shield; 2, a variable number of movable "body-rings" 

 or thoracic segments ; and 3, a caudal shield or "pygidium." 



The cephalic shield or buckler (figs. 103 107) is generally 

 more or less semicircular in shape, and is composed of a central 

 and two lateral pieces, of which the two latter may or may not 

 be united in front of the former. The central portion of the 

 cephalic shield is usually elevated above the remainder. It is 

 termed the " glabella," and it protected the region of the 

 stomach. The form of the glabella varies a good deal. 

 Usually it is widest in front (fig. 105), but its width may be 

 nearly uniform (fig. 106), or it may be widest posteriorly, and 

 contracted in front, as in Calymene (fig. 107). The glabella 

 is bounded at the sides by two grooves, which are known as 

 the " axal furrows," and is marked off behind by a third 

 groove, which is termed the " neck-furrow." The surface of 

 the glabella may be quite smooth, but it is ordinarily divided 

 into lobes by grooves, which originate in the axal furrows, and 

 pass inwards towards the middle line (fig. 103, 2). These 



L 



