A EEVIEW OF THE TBIXOBI.TE8. 273 



head. The facial suture is, however, in some cases entirely marginal, 



and hence not apparent on the surface of the shield, as in the genus 

 Trinucleus. According to its directiou, consequently, the trilobites 

 might be arranged in four groups : 1. — with the facial suture 

 terminating at the base of the head-shield; 2. — at the angles; 3. — at 

 the sides : and i. — with the suture marginal. A classification of 

 this kind, however, if carried out too exclusively, would tend, as 

 founded on a single structural peculiarity, to various groupings and 

 separations of a more or less artificial character. 



The eyes, when present, are situated on each side of the buckler 

 in the line of the facial suture, where this, at least, occurs upon the 

 surface. They are sessile, but more or less elevated above the im- 

 mediate surrounding parts ; and either compound after one of two 

 types, or pseudo-compound : consisting in the latter case of simple 

 stemmata in merely approximate union, as in the genus Harpes. The 

 compound eyes in the family of the Phacopsida are covered by the 

 common cephalic test, but this is pierced with numerous small aper- 

 tures through which the transparent cornea projects.* This forms 

 the reticulated eye, properly so-named. In the other families possess- 

 ing compound eyes, the cephalic test gives place around the eye to 

 a common cornea, which exhibits, in comparison with the eyes of the 

 Phacopsidce, a very delicate reticulation The reticulated appearance 

 is caused by the underlying facets. 



Many species appear to have been entirely destitute of eyes, pro- 

 perly so-called ; but it may be questioned whether this deficiency, at 

 least in certain cases, was not compensated by the presence of isola- 

 ted ocelli, destroyed more or less by the process of fossilization, or 

 perhaps obliterated by age — as in some existing crustaceans — during 

 the lifetime of the animal. In the modern limulus, a crustacean 

 type having certain characters in common with the trilobites, a pair 

 of ocelli accompany the compound eyes. Indications of ocelli are, 

 I believe, actually traceable in some of the apparently eyeless trilo- 

 bites. Two small median points or tubercles, which may perhaps be 

 legitimately attributed to ocelli, occur, for instance, on the glabella 

 of many specimens of Trinucleus concentricus. On the other hand, 

 it is well known, that amongst some of the marine parasitic Crustacea, 

 to which again the trilobites are in a measure related, only the males 



• Babrahde. Owing to the incompleteness of most specimens, it is rarely that these cha- 

 racters can be observed. If the eye, however, be at all preser vod, a common magnifying 

 glass will show a remarkable difference between the strongly-facetted phacopsida?, and tho 

 more delicately reticulated forms. It may not. perhaps, be useless to add, that the aid of 

 the lens is almost invariably required Cor the proper observation of the faoial sutures and 

 other structural details. 



T 



