KONGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 34. \:<> 8. 17 



posterior cephalic börder in an uninterrupted arch ] quite as iri several of the adull Ole 

 nellidae of the oldesl Cambrian. It can be taken as granted that its origin is the same ae 

 in the Olenellidie though a< presenl the only evidence al hand is the small larva of Para- 

 doxides celandicus, which Linnarsson called Parad, aculeatus. 2 In this we see the anterior 

 pleuron or the facial ridge alone present, elongated downwards like the 9ame pleuron in 

 the figure (i of Olenellus asaphoides and terminating like this in a fine spine stretching 

 backwards outside the posterior börder. Of the second pair of pleura there is nothing 

 to be seen. This must then have been aborted at an earlier stage t han in the 

 Olenellidae. 



The connexion between the free cheeks and the raiddle part of the head has been 

 ver)' lax not only in the Paradoxidce, hut on the vvhole in nearly all Cambrian trilobites 

 with free cheeks. When the free cheek is dissevered it shows no trace of the ridge, there 

 is only a large seallop on the spöt where it embraced the ridge. In the Paradoxida) the 

 rim of the indenture and the ridge are in so close contact that there is not the least place 

 for an eye between them, as can be seen in the few specimens with a complete head. In 

 all oculate trilobites again without any exception the facial suture separates that part of 

 the eve which is the real visual organ with corneal facets, from the interiör often elevated 

 portion, opposite it, the so called palpebral lobe. The eye is always placed on the free 

 cheek, 3 the lobe again always on the fixed cheek of the head shield. No real eye exists 

 without the palpebral lobe, and, on the other hand, that part of the facial ridge which 

 låter in the development changes to a palpebral lobe, occurs alone without any eye in a 

 great number of Cambrian trilobites, and consequently these are blind and sucb is the 

 case with the Paradoxidae and a great number of the succeeding. 



There is not the least evidence to support the suggestion that the »ocular ridge» is 

 homologous, with the eye of Apus 4 and that the real crystal cones lay sunk beneath the 

 surface in a water sac». As we, for instance, in Peltura have an »ocular ridge» (= facial 

 ridge mihi) on the tixed cheek and opposite its posterior extremity, the »eye lobe», a 

 real eye with facets on the free cheek it is not likely to suppose that the »ocular ridge» 

 nor the »eye lobe» ever functioned as a visual organ or that two widely different sorts of 

 eyes were placed in closest vicinity opposite each other. 



It may be worth while here to remind of the great similarities, whether analogous 

 or homologous, in the formation of the superior surface of the head in the trilobites and 

 the embryons and the newly hatched larva of Limulus. The latter have the head shield 

 separated into tive portions, partly through a facial suture which, as in Peltura, Dalma- 

 nites and others, forms a continuous line around the glabella. This suture divides the 

 eye node in a similar way as in the trilobites, that is, sectioning it in two parts, of which 



1 G. F. Mattiiew had before me, as I now lind, pointed out this distinction in bis memoir [llustrations 

 of the fauna of St. John N:o IV, p. 163. When he speaks of the embryonic stage in this and other pas- 

 sages he evidently means »larval stages», as the embryonic stages of necessity must remain unknown to us. 



(Låter remark.) 



2 Om faunan i lagren med Paradoxides ölandicus (1877), p. 359, pl. 14, f. 11. 



3 Excepting in Harpes, which has no free cheeks. 



4 Bernard, The systematic position of the Trilobites. Qu. Journ. Geol. Soc. 1894. p. 411. 



