32 



LINDSTRÖM, VISUAL ORGANS OF THE TRILOBITES. 



zontal row of black döts. All this is probablv not of any structural value, only due to 

 låter changes. 



The remarkable genus Harpides from the lowest Lower Silurian belongs also to 

 this group and has beside the fixed peduncle a peculiar elongated ridge going from the 

 eyes to the lateral margins of the head 1 , a ridge which is also present in soine of the 

 true Harpes. 



As Beecheh has shown 2 the larva of Trinueleus possesses quite the same trans- 

 verse ridge with intumescent eyelike extremities, and although the smallness of the speei- 

 mens has not permitted to ascertain the presence of a true eye, it may be apposite to 

 suppose it on the homology with the eyes of Harpes, a genus with which Trinueleus is 

 related. But as well known, in the adult Trinuclei there is no trace of these ocular 

 ridges nor of real eyes so that Reede in his paper »Blind Trilobites» 3 numerates Trinu- 

 eleus amongst these. Out of the nine Scandinavian species of Trinueleus no less than seven 

 have a well marked little tuberele on each side of the glabella placed exaetly on the same 

 spöt where the larval Trinuclei had their much larger eyes placed. In this ease it may 

 be allowed to suppose that the tubercles are the direct successors of the larval eye and 

 that they are true ocelli. Reede seenis to be willing to regard them as possessing a 

 visual function. 4 Beecher 5 holds the eye nodules in the larva and the ocelli in the adult 

 to be identical. 



But, as Barrande has shown, 6 there is a certain species of Trinueleus, the larva of 

 which wants a facial ridge and eyes, as there also are several adidt forms without ocelli. 

 These have remained on a much ancestral stage, while the larva with eyes are more 

 highly developed in such species, where the adult have been subject to a retrograde de- 

 velopment. Ampyx and Dionide, though completely blind, evidently belong to this group, 

 and once, as is to be hoped, larval forms may be discovered showing their development. 

 In a certain wav Arethusina shows eharacters proper for this group, in having the straight 

 ocular ridge, quite as in Harpes, but eyes of the reticulate type and probablv prismatic. 

 It thus like Harpes eonserved an ancestral characteristic long periods since it had dis- 

 appeared in most of the other genera. 



Of the groups, in which Joh:s Muller 7 long ago classihed the Crustacean eyes, his 

 second »Hauptgruppe» (»Aggregate von einfaehen linsenhalten Augen») and the fourth »zusam- 

 mengesetzten Augen . . . faeettirto Hornhaut» the former corresponds with my third and the 

 latter with myt wo first divisions. In so far ;is the cornea and its facets or lenses are to be re- 

 garded, there is the greatest analogy with the Isopoda. In vertical sections of Sphueroma we 

 have the same sort of elongated, flattened biconvex lenses as in Sphaerophthalmus and others. 





1 Harpides breviceps cannot belong lo this urnus and is rather related to Erinnys SALTBR, as also 

 Matthew holds it. 



2 Structure and appendages of Trinueleus. 



:i Geol. Magazinc 1898. pp. 439, 493, 552. 



4 1. c. p. 447. 



5 1. c. p. 309. 



,; Syst. Sil. de Bohéme 1, pl. 30, tigs 41—50. 



7 In Merkels Archii L829 p. 16 and in Treviranus Zeitschrift fur Physiologie Hd IV p. !i7. 



