KONGL. sv. VET. AKADEMIENS HANDLINGAR. BAND 34. \:<» 8. 31 



this residue is a whitish ring close inside the interiör ring wall (1*1. VI t'. 2). I would suggest 

 that this curious conformation is due to the original structure of the lens, supposing thal 

 it in these crustaceans has been I < u i 1 1 upon the saine plan as in several other Arthro- 

 poda. In Cymothoa 1 and in Sphseroma 2 for instance the lenses are buill iip of thin 

 strata, which are parallel with the convex outside, so thal on the inferior surface of the l<-ns 

 they are arched downwards and od the superior side upwards, being riol strictly con- 

 centric. In the spiders they are constructed upon lliis same plan 3 perhaps more evi- 

 dently. If now in Phacops the lens consisted of such semiconcentric strata and the upper 

 nioiety has been destroyed, the rest must have taken the shape as we lind ii. It is 

 moreover peculiar that the destruction has been exactly similar in all lenses of thal 

 specimen. Can it be due to the circumstance that the power of resistance in the inf< 

 rior strata has been greater? 



In Phacops quadrilineata the lenses are more elliptic than in the former. On their 

 interiör surface beneath the spiny tufts mentioned large hexaedral prisms of clear calca- 

 reous spar issue, one prism for each lens (Pl. V fig. 38), having thus a very deceptive ap- 

 pearance, but no doubt of inorganic origin with the lenses as a basis for their crystalli- 

 zation, quite as in the Cystoids where the interiör often is converted into a mäss of cry- 

 stalline prisms, issuing from the interiör surface of the plates. 



III. Genera with stemmata and ocelli. 



In a little group that has retained larval or ancestral characters during a great part of 

 the paleozoic period, the genus Harpes stånds as a type. It has ranged from the ol- 

 dest LoAver Silurian, if we join the related Harpides, to the middle Devonian. From near 

 the top of the glabella, though not so inuch forward as the facial ridge of the blind 

 trilobites (Olenus, Liostracus etc.) a- straight ridge of much varying length stånds out 

 on both sides and at its extremity two or three globular stemmata with glossy surface 

 lie encased. Probably this ridge has the same origin as in the Olenidse, the more so, as 

 there are indications of an extensive circulatory system. On plate IV fig. 18 — 19 t hr 

 right hand ocelli of Harpes vittatus Bark. from Lochkov, Bohemia, are represented. They 

 are two, lying isolated near each other, quite globular with circular outline, smooth and 

 glossy as to exhibit a shining surface. Their size is 0,4 mm in diameter. Being cut 

 vertically in the direction of the longitudinal axis of the head they resemble elongated 

 hemispheres, convex on the exteriör surface, slightly eoncave on the interiör. The test 

 of the head lies between them as a saddle and covers them only partially and on the 

 outer sides they lie with their margins encased in the head shield. Seen in thin sections 

 of the right lens and magnined the whitish mäss is traversed by vertical, blacker streaks, 

 standing somewhat radiating towards the sides and cancellate. The other lens has a hori- 



1 Bullar Philos Transact. 1878, pt. II, pl. 46, fig. 12. 



2 Bellonci Atti dei Lincei, Memorie X 1881, pl. II„ fig. 11. 



3 See Grenacher pl. II t'. 18 Epeira. 



