The Facial Suture of Trilobites. 105 



Nevertheless, amid till these changes the occipital, and sometimes the 

 next segment in front of it, retain a close resemblance to a trunk 

 segment. 



The fixity or non-fixity of the number of cephalic segments cannot 

 be definitely settled until the ventral surfaces of many more 

 trilobites are known. Meanwhile it is significant that in such 

 widely separated genera as Triarthrus and Nareila x five pairs of 

 cephalic appendages occur and that the last pair in each case 

 resembles those in front more than it does the trunk appendages. 

 The existence of this condition in so primitive a form as Marella 

 suggests that the cephalic segments became marked off as cephalic 

 en bloc, and proves that the posterior limits of the head-shield 

 became defined and the number of its segments fixed at a very 

 early stage in the evolution of the order. 



In all trilobites, young or old, which possess eye-lines or eye-lobes 

 as well as a clearly segmented glabella, the line or lobe is related to 

 the palpebral segment that is the fifth from the posterior margin. 

 This is another fact which indicates that these five segments are 

 homologous in all typical trilobites, and that no new segment has 

 been added to the cephalon since definitive trilobites came into 

 existence. 



How many segments lie in front of the palpebral it is quite 

 impossible to say. An additional one is sometimes indicated on the 

 glabella. 3 This must be the ocular segment whose pleural portion 

 bears the visual area of the eye upon its hinder margin. The 

 segmentation of the region in front of this is at present merely 

 a matter for conjecture. 



The Position of the Eye. 

 The position of the eye seems to be very variable. In the early 

 stages of development of many trilobites it is first seen on the 

 margin. In later stages it shifts on to the dorsal surface, and in 

 the adult it may be quite close to the glabella and to the posterior 

 margin of the cephalon. These facts led Bernard 3 to regard the eye 

 as a lonely wanderer on the face of the trilobite, and to conclude 

 that it "had no fixed hereditary locus on the dorsal surface". But 

 in other animals an organ that wanders usually takes a train of other 

 organs with it, that is to say it retains its major morphological 

 relationships. Thus the eye of the flat-fish, Pleuronectes,* wanders 

 during development from one side of the head over the dorsal 

 surface on towards the other side, but throughout its wandering it is 

 accompanied by its full complement of muscles, nerves, blood-vessel, 

 and bones. That is to say, it retains a definite position in relation to 

 the general morphology of the head. Bernard unconsciously 

 recognizes this fact for the trilobites when he says, " The eye never 



1 C. D. Walcott, "Middle Cambrian Brachiopoda, etc.": Smithsonian 

 Misc. Coll., vol. Mi, pp. 192, 193, 1912. 



2 C. D. Walcott, " Olenellus, etc.": Smiths. Misc. Coll., vol. liii, p. 277, 

 pi. xli, fig. 9, 1910. 



3 1894, p. 420. 



4 Vide F. J. Cole & J. Johnstone, L.M.B.C, Memoirs, No. viii : Pleuro- 

 nectes, pp. 174 et seq., 1901. 



