In Dalm. vulgaris and also in Phacops quadrilineata there is as already before mentioned a peculiar structure beneath the lenses, consisting of narrow, threadlike, straight lines, twice as long as the lenses (Pl. III 49-50, pl. V fig. 38). In a horizontal section they are found to be irregular prisms closely packed. It can not be any structure peculiar to the eyes or the lenses, rather some parasitic growth added since the death of the animal. The lenses are in several specimens composed of clear calcareous spar. In others again they have been filled with a dark muddy calcareous rock excepting in the lower moiety where there is left a residue of the white spar, having in all lenses assumed a regular shape which I consider as organic (Pl. VI fig. 5). This spar covers the whole bottom and its upper rim is incrassated and bent inwards. In horizontal sections this residue is a whitish ring close inside the interior ring wall (Pl. VI f. 2). I would suggest that this curious conformation is due to the original structure of the lens, supposing that it in these crustaceans has been built upon the same plan as in several other Arthropoda. In Cymothoa[30] and in Sphæroma[31] for instance the lenses are built up of thin strata, which are parallel with the convex outside, so that on the inferior surface of the lens they are arched downwards and on the superior side upwards, being not strictly concentric. In the spiders they are constructed upon this same plan[32] perhaps more evidently. If now in Phacops the lens consisted of such semiconcentric strata and the upper moiety has been destroyed, the rest must have taken the shape as we find it. It is moreover peculiar that the destruction has been exactly similar in all lenses of that specimen. Can it be due to the circumstance that the power of resistance in the inferior strata has been greater? [Footnote 30: BULLAR Philos. Transact. 1878, pt. II, pl. 46, fig. 12.] [Footnote 31: BELLONCI Atti dei Lincei, Memorie X 1881, pl. II, fig. 11.] [Footnote 32: See GRENACHER pl. II f. 18 Epeira.] In Phacops quadrilineata the lenses are more elliptic than in the former. On their interior surface beneath the spiny tufts mentioned large hexaedral prisms of clear calcareous spar issue, one prism for each lens (Pl. V fig. 38), having thus a very deceptive appearance, but no doubt of inorganic origin with the lenses as a basis for their crystallization, quite as in the Cystoids where the interior often is converted into a mass of crystalline prisms, issuing from the interior surface of the plates. III. =Genera with stemmata and ocelli.=