Since the above was written, Troedsson (1918, p. 57) has described the development of _Dalmanites eucentrus_, a species found in the Brachiopod shales (Upper Ordovician) of southern Sweden. This species follows a course similar to that of _D. socialis_, so that the full series of stages need not be described. The pygidium is, however, of especial interest, for there is a stage in which it shows two more segments than in the adult. Troedsson figures a pygidium 1.28 mm. long which has eight pairs of pleural ribs, while the adult has only six pairs. The ends of all these ribs are free spines, and were the development not known one would say that this was a case of incipient fusion, while as a matter of fact, it is incipient freedom. A further interest attaches to this case, because of the close relationship between _D. eucentrus_ and _D. mucronatus_. The latter species appears first in the _Staurocephalus_ beds which underlie the Brachiopod shales, so that in its first appearance it is somewhat the older. The pygidium of the adult _D. mucronatus_ is larger than that of _D. eucentrus_, having eight pairs of pleural ribs, the same number as in the young of the latter. In short, _D. eucentrus_ is probably descended from _D. mucronatus_, and in its youth passes through a stage in which it has a large pygidium like that species. Once more it appears that the small pygidium is more specialized than the large one. The full ontogeny of _Cyclopyge_ is not known, but young specimens show conclusively that segments are not transferred from the thorax to the pygidium, but that the opposite occurs. As shown by Barrande (1852) and corroborated by specimens in the Museum of Comparative Zoology, the process is as follows: The third segment of the adult of this species, that is, the fourth from the pygidium, bears a pair of conspicuous cavities on the axial portion. In a young specimen, 7 mm. long, the second segment bears these cavities, but as the thorax has only four segments, this segment is also the second instead of the fourth ahead of the pygidium. The pygidium itself, instead of being entirely smooth, as in the adult state, is smooth on the posterior half, but on the anterior portion has two well formed but still connected segments, the anterior one being more perfect than the other. These are evidently the two missing segments of the thorax, and instead of being in the process of being incorporated in the pygidium, they are in fact about to be cast off from it to become free thoracic segments. In other words, the thorax grows through the degeneration of the pygidium. That the thorax grows at actual expense to the pygidium is shown by the proportions of this specimen. In an adult of this species the pygidium, thorax, and cephalon are to each other as 9:11:13. In the young specimen they are as 10:6:12, the pygidium being longer in proportion both to the thorax and to the cephalon than it would be in the adult.