16 LINDSTRÖM, VISUAL ORGANS OF THE TRILOBITES. 



its pleural nature, as the thoracic pleura comrnonly are dividcd through such grooves. 

 The same peculiarity is also observable in several of the American Olenellidae. It is 

 much the same with the posterior pleuron, the pleural nature of which is revealed through 

 its spine, that is homologous and identical with the spiny terminations of the thoracic 

 pleura. We have thus through the remarkable finds of Ford and Walcott combined 

 received an explanation of the morphological origin and nature of the facial ridge, the 

 so called eve lobe and found that it has nothing whatever of the character of a visual 

 organ. But it must be borne in mind, that these developmental changes are peculiar 

 only to the Olenellidaj, the origin of the facial ridge in the låter trilobites is, as we shall 

 see, quite a different one. 



The Olenellida) belong chiefly to the oldest of the Cambrian beds with trilobites, 

 and none of them has as yet been found higher up. 



Next in order of evolution we have the important tribe of the Paradoxidae. These 

 are preeminently distinguished from the 01enellida> through the well developed facial 

 suture, which without exception in them all runs outside the facial ridge and separates 

 this from the free cheek. 1 This is a great step forwards in the evolution and establishes 

 the fact, demonstrable also by other evidence, that the formation of the facial suture is 

 subsequent to the appearance of the facial ridge. This preexisting ridge seems to have 

 had no small influence on the development of the suture, it checked its progress from 

 the front or from the sides toward the fixed cheeks and directed its course against the 

 genal angles. It lay as a protection for the glabella against this disseverance, causing 

 the separating line to run along its outside. 



This group consists of the genus Paradoxides proper, as well as of Centropleura, 

 Metadoxides and Hydrocephalus, if this is an adult form and not the larva of an un- 

 known Paradoxides. Perhaps such forms as »Conocephalites» Emmrichi Barrande, as well 

 as Anomocare limbatum, An. excavatum, Bathyuriscus and Dolichometopus may on account 

 of the shape of their facial ridges be considered as related to the Paradoxidae. But this 

 must be left for coming researches to decide. Some American Cambrian forms also share 

 in this characteristic and may upon closer inspection be ranged here. So Zacanthoides. 

 In these as in the true Paradoxida? the facial suture follows the ridge along its whole 

 length, while in the trilobites of the third group the facial suture is in contact only with 

 the posterior end of the ridge, the so called eye lobe. Remopleurides does not show 

 characters, that as Beecher thinks, could unite it with the Paradoxida?. These are blind 

 and Remopleurides has well developed eyes and an organization that gives it an isolated 

 position in the system. 



The facial ridge continues in a great variety of shapes, short or long, but always 

 forming the fraction of a circle, always of nearly equal thickness, only slightly tapering 

 towards one of the extremities, and always when in direct connection with the glabella, 

 starting from the base of its foremost, largest segment. As a rule the ridge is more 

 developed in the young or larval individuals, continuing from the glabella to near the 



1 They bave thus a quinquepartite cephalic shield, as the låter trilobites. 





