20 LINDSTRÖM, VISUAL ORGANS OF THE TRILOBITES. 



ing from it have been reduced in size and number, but are still visible. Thus, if I am 

 right, the two, the facial ridge and the raain trunc, are in causal connection and the 

 former has been moulded by the låter, when it had gained in size sufficiently, and when 

 a richer affluxion of nutritive fl nid was directed baekwards towards the point where 

 the eye at last originated. The further changes in this ridge thus are related to the 

 development of the eye. As far as I am aware, there are as yet no data to tell us 

 whether the appendages or pleura of the glabella in the larval (or primordial) Olenellidie 

 have the same origin as the ridge of the Olenidie or are homologous with it. Perhaps 

 it ma v be so in respect to the fo remost one, which also is persisting. It is, however, 

 not known how the pleura in the olenellid glabella have been developed, whereas it is 

 well ascertained by the growth of the glabella in Sao, Liostracus and others that such 

 pleura never have been developed in them. 



What the phylogenetic evolution has taught us concerning the formation of the 

 facial ridge is confirmed by the ontogenetic development of individuals of some speeies 

 as for instance Sao hirsuta Barr., Liostracus sp. Brög. and others. 



This development can be followed in detail through the excellent tigures of the 

 larval stages of Sao hirsuta which Barrande has given in his magnificent work, vol. I, 

 pl. 7. In the Jirst stage iigured (tigs. 1 d — e) the whole animal consists only of the 

 head shield which is completely smooth, the glabella scarcely segmented, no facial ridge, 

 no facial suture. In the second stage, according to me (tigs. 2 — 4 a), the glabella has 

 become distinctty segmented, and the pygidium and partly the thorax have been added. 

 In the third stage, 4 c, d — 9, one small narrow string exits from each side of the front 

 of the glabella, making the first faint beginning of the facial ridge. They form a right 

 angle with the glabella. In continued growth they beeome by degrees a little more 

 curved (tigs. 5 c, G b) and the lengthened ridge bends parallel to the outer lateral margin 

 of the head (tig. 9 b). In tig. 9 d it has become so far complete that it reaches nearly 

 back to the posterior cephalic margin, but is still of the same narrowness all along. 

 In the fourth stage, in a specimen (tig. 10 a) of 3 mm. in length, the facial suture makes 

 its first appearance, setting in from the posterior margin of the head and meeting in its 

 forward progress the posterior extremity of the facial ridge which now begins to swell 

 out. It seems that both have a mutual influence on each other, the suture being de- 

 viated from a straight course 1 to take a bend outside the ridge, and the extremity of 

 the ridge again at this contact to increase in size so as to form the elongated tubercle, often 

 called palpebral lobe (hg. 14 b). This now augments in the same råte as the whole body. 



The characters of the four stages of the development of the larva consequently 

 are: l:o The archaic stage, only head shield with ridgelike glabella- 2:o The coming and 

 growth of the pygidium and the complete segmentation of the glabella. 3:o The coming 

 and growth of the facial ridge. 4:o The eoming and development of the facial suture. 



Barrande regards the whole ridge as a prolongation of the eye and the tubercle 

 at its posterior extremity as the eye itself. 2 But, again, in page 399 he says »Sa surface 



1 In Trinucleus and Ampyx where there is no facial ridge, the suture has a straight direction along 

 the lateral margins. 



- Page 383 1'ceil arqué est prolongé par un ti let en relief, vers le front de la glabelle.. 



