170 



THE CHICAGO ACADEMY OF SCIENCES. 



FIGURE i. The cephalon of a trilobite: r, 

 glabella; a/, axial or dorsal furrows; fxc, fixed 

 cheeks; fc. free cheeks; s, facial suture; ft, 

 frontal lobe of glabella; 1, 2, 3, first, second 

 and third lateral glabellar furrows; /*, / 2 , /', first, 

 second and third lateral glabellar lobes; of, occi- 

 pital furrow; os t occipital segment; p, palpebral 

 or ey e lobes; ,, visual surf ace of eye;m/ margin- 

 al furrow; mb t marginal border; gs, genal spines. 



two portions by the* 

 facial sutures, and 

 among the different 

 orders, families and 

 genera, these sut- 

 ures are exceedingly 

 variable in position 

 and direction and are 

 of great taxonomic 

 value. The portion 

 of the cheeks lying 

 between the facial 

 sutures and the dorsal 

 or axial furrows are 

 known as the fixed 



cheeks, and ,the out- 



er portions as the free cheeks. The compound eyes, when 

 present, are always situated upon the free cheeks adjacent 

 to the facial sutures; the lobe-like extension of the fixed 

 cheek adjacent to the inner margin of the eye is known 

 as the palpebral lobe or eye lobe. The portions of the facial 

 sutures lying in front of the eyes are known as their anterior 

 limbs, while the portions back of the eyes are their posterior 

 limbs. The anterior limbs of the facial sutures may unite and 

 pass around the anterior end of the glabella, or they may remain 

 separate, in which case they terminate at the anterior margin 

 of the head ; their posterior limbs may terminate at the posterior 

 margin of the head, near the genal angles or upon the lateral 

 margins of the head. That portion of the cephalon lying be- 

 tween the facial sutures, comprising the glabella and the fixed 

 cheeks, is known as the cranidium. It frequently happens 

 that^the free cheeks have become separated before fossilization 

 takes place, in which case the specimen consists of the cranidium 

 alone. In many cases the marginal portion of the head, some- 

 times only laterally and again laterally and anteriorly, is dif- 

 ferentiated from the general surface as a marginal border; 

 this may be flattened, convex or concave, and may vary greatly 

 or may be entirely absent. The furrow or groove limiting 

 the marginal border internally is the marginal furrow-. Fre- 

 quently the cheeks are traversed near their posterior margins 

 by .furrows which originate at or near the lateral terminations 

 of the occipital furrow of the glabella, and continue in a di 



