18 LINDSTRÖM, VISUAL ORGANS OF THE TEILÖBITES. 



one adheres to the central fixed cheeks and the other to the free cheeks. 1 And to judge 

 by the figures of Kingsley the former, the whitish moiety is the first developed and 

 sometimes for a while quite alone as the facial ridge of the trilobites and probably also 

 anterior to the suture, as this is not complete at this stage. This white node reniinds 

 of the small facial ridge in Arionellus ceticephalus Barr. 



As Kishinouya 2 has already ])ointed out and as I have anticipated above the head 

 of the most developed trilobites in their adult state, and the head of the larval Liinulus 

 consists of tive parts, viz. 1) the glabella in the centre, 2 & 3) the fixed cheeks, 4 & 5) 

 the free cheeks. An elevated ridge in the adult Limulus shows where the suture once 

 lay and it is on the outside thereof that the eye of the adult is placed. What other 

 authors call the ridge or the eye ridge in Limulus, Kishinouya rightly names a suture. 



It was the renowned Swedish naturalist Wahlenberg, who first recognized the im- 

 portance of the facial suture, which he called »linea ocularis», 3 but to another Swedish 

 palasontologist Dalman 4 the exact definition of this suture is due, to which he gave the 

 nanie still in use. He expressly remarks that the suture crosses the ocular node and 

 limits the outside of the »lobus palpebralis» and he makes a clear distinction between 

 that lobe and the »tuberculi and eminentiae oculares» (= facial ridge) of which he says 

 that they are the more or less evident elevations situated in the blind Palaeades on the 

 place of the eyes (in which he is wrong) »and which perhaps are an indication of such 

 organs». But then he says doubtfully (p. 255) »tuberculorum ocularium veram naturam 

 determinare haud ausi sumus, etsi oculorum formam sat bene exhibere videantur» and 

 he adds concerning Paradoxides »oculi nulli, eorurn loco autem tuberculi duo». 



Group 2. The Olenidae and related families. 



Next in the ascending order we have the largest group of trilobites in the Camb- 

 rian, of which the greatest part is formed by the Olenida3. A facial ridge different in 

 shape and different in developraent from that in the former group characterizes them. 

 Unlike the semilunate ridge of the Olenelliche it issues mostly from the front of the first 

 segment of the glabella and goes generally backwards till it meets the facial suture. It 

 is narrow and fine as a thread, but for the rest assumes a great variety of forms. It 

 may be curved as a circle segment as in Sao, Liostracus, it inay be long and straight, 

 standing out in a right angle to the glabella as in Eurycarc, it may be short and straight 

 in an acute angle to the glabella as in Parabolina and so forth. In the same genus, as 



1 Packard, Development of Limulus polyphemus. Memoirs Boston Soc. Nat. H. vol. II, pl. V, tig. 25. 

 Nötning is said about the exteriör structure of the eyes. 



ItoiiKN, Zur Embryologie und Morphologie des Limulus Polyphemus. Jenaische Zeitschrift 1871. A 

 very good figure (pl. XIV, t. 4) shows clearly the two parts of the eyes, the interiör one being larger. 



KlNCSLKY also (I)e\el. of I.inuilus, Journal of Morphology, vol. 7, 1892, pl. VI, fig. 34) has in the last 

 larval stage the suture and the eye in two parts, of which one is white ly ing inside the suture and the eye 

 proper, black, outside. 



2 Journal College of Science Tokio, vol. V, p. 5:i, 1892. 



3 Additamenta qusedam ad petrificata telluris Suecana, in Acta Upsaliensia, vol VIII, 1821, p. 294. 

 ' Vet.-Akad. I Ian.il. 1826 p. 126. 





