CRUSTACEA— TRILOBITA. 251 



(posterior) furrow is the occipital furrow and bounds anteriorly 

 the occipital ring which is just in front of the first segment of the 

 thorax. On either side of the glabella is a pair of cheeks, divided 

 by the facial suture into fixed cheeks (those next to the glabella) 

 and free cheeks (the outermost or movable portion). The latter 

 are often prolonged into genal spines. The compound eyes are sit- 

 uated on the free cheeks and they are overshadowed by more or 

 less prominent eyelids or palpebral lobes, which are lateral lobes 

 from the fixed cheeks. The facial suture thus passes between the 

 eyes and the palpebral lobes, and when, as is often the case, the 

 free cheeks become separated after the moult or death of the 

 animal, only the palpebral lobes remain on the central portion of 

 the cephalon (cranidium) to indicate the former position of the 

 eyes. The anterior end of this palpebral lobe is often bound to 

 the glabella by the ocular ridge. The border of the cephalon is 

 often distinctly marked and is spoken of as the cephalic limb. At 

 the margin, it is folded down and back, making the doublure, which 

 continues backwards, and often produces hollow or solid genal 

 spines. To the anterior lower portion of the doublure is attached 

 the lip or hypostoma, which is often found separate; this is ho- 

 mologous with the upper lip of other crustaceans. 



The middle portion of the carapace, or thorax, consists of a 

 varying number of divisions or segments articulated with each 

 other, and commonly permitting the enrollment of the animal. 

 Each segment of the thorax is divided by two furrows into a 

 middle portion (axis) and two lateral divisions (pleura). 



The posterior portion of the carapace, the abdomen or pygidium, 

 consists of a single piece comprising a central axis and lateral 

 lobes. The axis and the lobes commonly show transverse furrows, 

 similar to the divisions of the thorax, and they are often so strongly 

 marked that a line of division between thorax and pygidium is 

 difficult to determine. 



Probably all trilobites had jointed appendages, which included 

 antennce, mouth parts, legs and gill fringes, comparable in a general 

 way to those of the lower orders of modern Crustacea. These are 

 shown in the restoration of Triarthrus becki (Fig. 1590). 



