30 VISIBLE STRUCTURE OF THE TRILOBITES. 



been absolutely lost. But considerations of this kind belong to the following chapter, since 

 I merely intended to describe the immediately recognizable structure in the present chapter, 

 and this I believe I have now accomplished as completel}' and generally as possible.* 



* The lobes of the cephalic protuberance, even if the}- are not complete divisions, might at once 

 be termed rings ; the anterior being denominated frontal or antennary ring ; the second, the ophthal- 

 mic ring ; the third, the gill ring ; and the fourth, the labial ring ; to which latter the accessory parts 

 of the mouth were affixed. The transverse protuberance at the posterior margin, which is constant!}' 

 present, and has aheady been mentioned as the articular border, might also be termed the arti- 

 cular ring. 



