232 TRILOBITA CHAP. 
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 Lindstrém 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 Olenellus, 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 off 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 
Lindstroém’s view no trace of an eye 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 Lurycare 
found in the Olenus beds of the Upper Cambrian.  Sphaeroph- 
thalmus and Ctenopyge, found in the higher beds of the Cambrian, 
also possessed eyes, but Olenus and Parabolina were probably 
blind. 
On the ventral surface of the head there is a flat rim around 
the margin (Fig. 137, B, 6); 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 (¢, ¢’) 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 Phacops and Remopleurides, the two facial sutures unite on 
the dorsal surface, in front of the glabella, the median part of 
