The appeal to the ontogeny does not give as full an answer to this question as could be wished, for the complete life-history of no trilobite with a large pygidium is yet known. While the answer is not complete, enough can be gained from the study of the ontogeny of Dalmanites and Cyclopyge to show that in these genera also the thorax grows by the breaking down of the pygidium and that no segment is ever added from the thorax to the pygidium. The case of Dalmanites socialis as described by Barrande (1852, p. 552, pl. 26) will be taken up first, as the more complete. The youngest specimen of this species yet found is 0.75 mm. long, the pygidium is distinctly separated from the cephalon, and makes up 25 per cent of the length. This is probably not the form of the shell as it leaves the egg. At this stage there are two segments in the pygidium, but they increase to four when the test is 1 mm. long. The cephalon has also increased in length, however, so that the proportional length is the same. The subjoined table, which is that compiled by Barrande with the proportional length of the pygidium added, is not as complete as could be desired, but affords a very interesting history of the growth of the caudal shield. The maximum proportional length is reached before the introduction of thoracic segments, and during the appearance of the first five segments the size of the pygidium drops from 25 to 15 per cent. Several stages are missing at the critical time between stages 8 and 9 when the pygidium had added three segments to itself and has supplied only one to the thorax. This would appear to have been a sort of resting or recuperative stage for the pygidium, for it increased its own length to 20 per cent, but from this stage up to stage 12 it continued to give up segments to the thorax and lose in length itself. After stage 12, when the specimens were 8 mm. long, no more thoracic segments were added, but new ones were introduced into the pygidium, until it reached a size equal to one fifth the entire length, as compared with one fourth in the protaspis.
| Stage | Length in mm. | Percentage of pygidium | Segments in thorax | Segments in pygidium |
| 1 | 0.75 | 25 | 0 | 2 |
| 2 | 0.75 | 25 | 0 | 3 |
| 3 | 1.00 | 25 | 0 | 4 |
| 4 | 1.00 | 22 | 1 | 3 |
| 5 | 1.25 | 20 | 2 | 3 |
| 6 | 1.25 | 18 | 3 | 3 |
| 7 | 1.60 | 15 | 4 | 3 |
| 8 | 1.60 | 15 | 5 | 3 |
| 9 | 3.00 | 20 | 6 | 6 |
| 10 | 3.50 | 20 | 7 | 6 |
| 11 | 8.00 | 18 | 9 | 7 |
| 12 | 8.00 | 16 | 11 | 5 |
| 13 | 12.00 | 16 | 11 | 7 |
| 14 | 19.00 | 18 | 11 | 9 |
| 15 | 95.00 | 20 | 11 | 11 |
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.
This conception of the breaking down of the pygidium to form the thorax will be very helpful in explaining many things which have hitherto seemed anomalous. For instance, it indicates that the Agnostidæ, whose subequal shields in early stages have been a puzzle, are really primitive forms whose pygidia do not degenerate; likewise the Eodiscidæ, which, however, show within the family a tendency to free some of the segments. The annelidan Mesonacidæ may not be so primitive after all, and their specialized cephala may be more truly indicative of their status than has previously been supposed.
The facts of ontogeny of trilobites with both small and large pygidia do show that there is a reduction of the relative size of the caudal shield during the growth-stages, and therefore that the large pygidium in the protaspis is probably primitive. The same study also shows that the large pygidium is made up of "coalesced segments" only to the extent that they are potentially free, and not in the sense of fused segments.
That the narrow type of axial lobe is more primitive than the wide one has already been demonstrated by the ontogeny of various species, and space need not be taken here to discuss the question. Most Cambrian trilobites have narrow axial lobes even in the adult so that their development does not bring this out very strikingly, though it can be seen in Sao, Ptychoparia, etc., but in Ordovician trilobites such as Triarthrus and especially Isotelus, it is a conspicuous feature.