20 VISIBLE STRUCTURE 



has, relatively speaking, the largest e)'es among the Triloliites, consequently also the largest 

 lenses and the thinnest cornea : a fact which is decidedly cstalilishcd, and suSiciently explains 

 the absence of a smooth appearance in the eyes of this genus.* The truth of this view is, 

 however, still further supported by the circumstance that the facettes in all the Crustacea, and 

 in most of the Articulata with a facetted cornea, are perfectly contiguous, leave no open in- 

 terstices, and individually are much less convex than in Phacops. The eye of the latter genus, 

 if it possessed a facetted cornea, could only be compared with the eyes of some nocturnal 

 insects, for instance, of the Redavies, or of some of the parasites, such as the RMpUdoptera, 

 in which the facettes are larger, more strongly arched, and situated more remotely from each 

 other, or it might be explained as an aggregate of simple eyes ; against which conjecture, 

 however, there would always be the fact of their peculiar circumscribed form. Aggregates 

 of simple eyes, as they occur in Mjnopock, and in some of the hopods, consist however 

 always of a smaller number of ocelli, whilst the number found in Phacojjs is very considerable.f 

 Thus I beheve I have proved the correctness of my assertion, that this genus, in common with 

 all other Trilobites, possessed a smooth cornea. Before concluding this part of the subject 

 I might, however, refer to the frequent actual deficiency of the cornea and lenses in Cahjmene 

 BlumenhacJiii, and quote the absence of the latter, which naturally arose from the small size, 

 as an additional argument in favour of my view. These parts of the eye were, indeed, so 

 small in this instance, and their coats so tender, that they could not have been petrified 

 after the loss of the protective horny cornea membrane. 



SECTION VIII. 



We shall now return to the already mQwiwweA liiiea facinlis, or sidiira temporalis, and trace 

 the principal variations of these lines. There can certainly be no doubt that the possession of 

 a temporal suture is a common character of all Trilobites ; it exists equally in Paradoxides and 

 Olenus. It is generally first recognized at the anterior margin of the common cephalic shield, 

 at a moderate distance from its centre, so that both lines remain at a rather greater distance 

 from each other than the transverse diameter of the cephalic protuberance at its broadest part. 

 But in Ogijqia, Phacops, Homalonoins, and AsapJuis, the temporal sutures extend themselves at 

 the anterior margin to the extreme point of the cephalic shield, and here unite, forming an 

 arch or angle. In the other genera they are curved on the lower side, round the anterior 

 margin of the head, and terminate in the margin which separates the cephalic shield from the 

 mouth. Converging a little towards the posterior part, they now approach from the margin to 

 the protuberance, as far as the region where the eyes are situate, here they describe the outward 

 curved lobe over the eye (operculum oculi) already alluded to, and again extend behind it 

 rather more towards the outer part to reach the margin of the cephalic shield at a second point. 



* Since the publication of tlie German edition, I have observed in the eyes of Phacops mucronatus, 

 from Bohemia, the globular spots hollowed out like a funnel, all regularly in the same manner ; a form 

 impossible, I think, if the spots were coruese. 



t I counted 163 hemispheres of leuses in each C3'e of the Phacops arachnoides ; of Phacops 

 mucronatus, on the other hand, 209; of Phacops lat'ijrons, only 90- 100. 



