In the particular case of the Trinucleidæ, which were burrowers, the spine is present on only the oldest and most primitive of the group, a form which has only a most rudimentary fringe. It is obvious from the large size of the pygidium in the larval trinucleid that this family is derived from a group of free swimmers. _Trinucleoides reussi_ was perhaps in the transitional stage, just leaving the swimming mode of life, and belonged to a group which had not developed any other "statocyst" than the median spine. Among the later Trinucleidæ the spine became a vestigial tubercle, and in some cases entirely disappeared. A similar history can be traced in the Cheiruridæ, starting from some such forms as the American Lower Ordovician _Nieszkowskia_ (_N. perforator_ p. ex.). Another example of a median spine instead of a tubercle is in Goldius rhinoceros (Barrande). Since this species is not from the oldest Goldius-bearing rocks, but from the Lower Devonian, it does not follow what seems to be the general rule, but makes an interesting exception. Goldius rhinoceros (Barrande) (Supplement, 1872, pl. 9, figs. 12, 13) has the median tubercle elevated into a stubby, recurved spine very suggestive of the horn of a rhinoceros. Since the eyes of this species are very well developed, there seems no especial reason for the elevation of a parietal eye, and the example certainly does not support that interpretation. 3. This tubercle is essentially similar to other tubercles on the median line of cephalon, thorax, and even pygidium. This has been discussed sufficiently under section 1 above, but it may perhaps be justifiable to point out that in some of the Agnostidæ there is a median tubercle on both shields, and since it has not yet been demonstrated beyond question which shield is the cephalon, to say which one is a parietal eye and which one is a tubercle is impossible. In other words, the parietal eye can not be differentiated from any other tubercle except by its position. 4. One of the as yet unexplained features of the protaspis of trilobites is the absence of the "nauplius eye." Beecher (1897 B, p. 40) explained this on the ground of the extremely small size of the protaspis and the imperfection of the preservation. If the median tubercle were really a median eye, it should be present in the protaspis and the earlier stages of the ontogeny, even if not in the adult, and should certainly appear before the compound eyes. (In _Limulus_, however, the compound eyes appear first.) The median eye has not so far been seen in any young trilobite in any stage previous to that in which compound eyes are present. The full ontogeny is not known of any species with compound eyes in which the median tubercle is present in the adult, but theoretically the median eye should be most prominent in the young of just those primitive trilobites about whose development most is known. NERVOUS SYSTEM.