232 TRILOBITA 



Many of the members of these families possess a lobe closely 

 resembling a palpebral lobe, and a corresponding excavation in 

 the free cheek; such forms have been generally regarded as 

 possessing eyes ; and the absence of any indication of lenses in 

 those cases, on which Lindstrom lays stress, has been explained 

 by the comparatively imperfect preservation of these early 

 Trilobites. The development of the supposed eye-lobe in some 

 of the Paradoxidae and Olenidae differs from that of the eyes in 

 other families of Trilobites. In the latter the eye appears first 

 at the margin of the head and always in connexion with the 

 facial suture. But in Ohnellus, in which there is said to be no 

 facial suture, development shows that the crescentic eye-like lobe 

 (Fig. 145, E) is really of the nature of a pleura coming oS from 

 the base of the first segment of the glabella. In Paradoxides, 

 which resembles Olenellus in many respects, a facial suture is 

 present and forms the outer boundary of the eye-like lobe, but it 

 is developed subsequently to the appearance of the latter, which 

 seems to be similar to that of Olenellus. In some genera of the 

 Olenidae the eye-line, which comes off from the first segment of 

 the glabella, ends in some cases in a swelling or knob which has 

 hitherto been regarded as a palpebral lobe, but according to 

 Lindstrom's view no trace of an eje has been found in connexion 

 with that lobe, nor is there any space between the lobe and the 

 free cheek in which the eye could have occurred. If this view 

 is correct it follows that the majority of the Cambrian Trilobites 

 were blind. The earliest genus with eyes would then be Eurycare 

 found in the Olenus beds of the Upper Cambrian. Spliaeroph- 

 thcdmus and Gtenopyge, found in the higher beds of the Cambrian, 

 also possessed eyes, but Olenus and Paraholina were probably 

 blind. 



On the ventral surface of the head there is a flat rim around 

 the margin (Fig. 137, B, I); this rim or "doublure" is the 

 reflexed border of the cephalic shield. In many Trilobites its 

 median part in front is cut off by sutures so as to form a separate 

 plate (e) ; such is the case when the two facial sutures (c, c') cut 

 the anterior margin of the cephalic shield and are continued 

 across the doublure, where they are joined by a transverse or 

 rostral suture {d) just below the margin. When, however, as 

 in Phaco'ps and Bemopleurides, the two facial sutures unite on 

 the dorsal surface, in front of the glabella, the median part of 



