_Sao_, _Liostracus_, _Ptychoparia_, and _Solenopleura_ all have the same sort of protaspis. In all, the axial lobe reaches the anterior margin and is somewhat expanded at that end; in all, the glabella shows but slight trace of segmentation; and in all, the pygidium occupies from one fifth to one fourth the total length. There is considerable variation in the width of the axial lobe. It is narrowest in _Ptychoparia_, where in the middle it is only 14 per cent of the whole width, and widest in _Solenopleura_, where it is 28 per cent. In _Ptychoparia_ the pygidium of the protaspis occupies from 18 to 22 per cent of the whole length. In the adult it occupies 10 to 12 per cent. In _Solenopleura_ it makes up about 26 per cent of the protaspis, and in the adult about 8 per cent. In the youngest stages of all these trilobites, the pygidium is incompletely separated from the cephalon. The first sign of segmentation is a transverse crack which begins to separate the cephalon and pygidium, and by the time this has extended across the full width the neck segment has become rather well defined. In this stage the animal is prepared to swim by means of the pygidium, and first becomes active. The coincident development of the free pygidium and the neck-ring strongly suggests that the dorsal longitudinal muscles are attached beneath the neck-fur row. The single protaspis of _Paradoxides_ now known, while only 1 mm. long, is not in the youngest stage of development. It is like the protaspis of _Olenellus_ in having large eyes on the dorsal surface and a narrow brim in front of the glabella. The glabella is narrower than in the adult. The initial test of no agnostid has probably as yet been seen, as all the young now known show the cephalon and pygidium distinctly separated. _Phalacroma bibullatum_ and _P. nudum_ are both practically smooth and isopygous when 1.5 mm. long. _P. bibullatum_ shows no axial lobe at this stage, but a wide glabella and median tubercle develop later, and when the glabella first appears, it extends to the anterior margin. In _Peronopsis integer_ and _Condylopyge rex_, the axial lobe is outlined on each of the equal shields in specimens about 1 mm. long, but is without furrows and reaches neither anterior nor posterior margin.