OF CENTRAL CANADA PART IV. 



253 



&c.) it is contracted in that direction. In some, again, (Triarthrus, 

 &c.), it is of nearly uniform width throughout. Very prominent, 

 also, in some genera ; and in others, but feebly elevated and compara- 

 tively inconspicuous. The head-shield exhibits likewise, in most 

 examples, on each side of the glabella a sutural line (=f. f. in Fig. 

 1 66) called the " facial suture." The hinder extremity of this sutural 

 line terminates either at the posterior margin of the head-shield ( asa- 

 phus, &c.); or at the angles of this (calymene, &c. ); or at the sides, near 

 the level of the eyes (Phacops, Ceraurus, &c.). In some few genera, 

 however, the facial suture is not preceptible. The eyes, when present, 

 occur on each side of the head-shield in the line of the facial suture, 

 just on the outer edge of this. In all genera but Harpes they are 

 compound, as in ordinary crustaceans, insects, &c., and in certain 

 genera ( Dalmanites, Phacops), the component facets are comparatively 

 large, forming the so-called " reticulated eye." The outer sides or 

 " cheeks " (gena) of the head-shield often separate along the line of 

 the facial suture, and are found detached. The shell is continued 

 over the nead-shield, and a so-called labrum or hypostoma is occasion- 

 ally found, where the mouth was situated, on the underside. This 

 is commonly found detached, and generally oval in form ; but in the 

 genus Asaphus it presents a forked or horse-shoe shape, with the 

 concavity at its lower or posterior margin. 



The body or thorax of the trilobite consists of a series of movable 

 rings or segments, varying in number, according to the genus or the 

 state of development of the individual. Two, generally deeply- 

 marked, longitudinal grooves or furrows traverse the thorax and 

 extend into the head-shield above and the pygidium below. In the 

 head-shield they limit the glabella. They divide the thorax into 

 three parts a central part or axis, and two side portions, or pleurae 

 ( = a, and pi. of figure 166.) These latter have their free ends 

 rounded in some species, and pointed or even prolonged in part or 

 wholly into spines, in others. In some genera (e. g. ceraurus or 

 cheirurus) there is a raised band on each pleura, and in others a 

 narrow groove. The degree of mobility with which the thoracic 

 segments were endowed, at least in most cases, enabled the trilobite 

 to bring the under parts of the caudal and head shield together, 

 both for the protection of the branchial feet or more or less 

 undefended portions of the body, and also, in all probability, to 



