Among the trilobites of the Lower Cambrian, no very young have been seen except of Mesonacidæ. Of these, the ontogeny of _Elliptocephala asaphoides_ Emmons is best known, thanks to Ford, Walcott, and Beecher, but, as the last-named has pointed out, the actual protaspis or earliest shield has not yet been found. The youngest specimen is the one roughly figured by Beecher (1895 C, p. 175, fig. 6). It lacks the pygidium, but if completed by a line which is the counterpart of the outline of the cephalon, it would have been 0.766 mm. long. The pygidium would have been 0.183 mm. long, or 23 per cent of the whole length. The axial lobe was narrow, of uniform width along the cephalon, showed a neck-ring and four indistinct annulations, but did not reach quite to the anterior end, there being a margin in front of the glabella about 0.1 mm. wide. The greatest width of the cephalon was 0.66 mm., and of the glabella 0.233 mm., or practically 35 per cent of the total width. Other young _Elliptocephala_ up to a length of 1 mm., and young _Pædeumias_, _Mesonacis_, and _Holmia_ (see Kiær, Videnskaps, Skrifter, 1 Mat.-Naturv. Klasse, 1917, No. 10) show about the same characteristics, but all these have large compound eyes on the dorsal surface and specimens in still younger stages are expected. It may be pointed out, however, that in these specimens the pygidium is proportionately larger than in the adult. Walcott cites one adult 126 mm. long in which the pygidium is 6 mm. long, or between 4 and 5 per cent of the total length, while in the incomplete specimen described above, it was apparently 23 per cent. In a specimen 1 mm. long figured by Walcott, the pygidium is 0.15 mm. long, or 15 per cent of the whole length. The development of several species of trilobites from the Middle Cambrian is known. Barrande (1852) described the protaspis of _Sao hirsuta_, _Peronopsis integer_, _Phalacroma bibullatum_, _P. nudum_, and _Condylopyge rex_. Broegger figured that of a _Liostracus_ (Geol. For. Förhandl., 1875, pl. 25, figs. 1-3) and Lindstroem (1901, p. 21) has reproduced the same. Matthew (Trans. Roy. Soc. Canada, vol. 5, 1888, pl. 4, pls. 1, 2) has described the protaspis of a _Liostracus_, _Ptychoparia linnarssoni_ Broegger, and _Solenopleura robbi_ Hartt. Beecher (1895 C, pl. 8) has figured the protaspis of _Ptychoparia kingi_ Meek, and the writer that of a Paradoxides (Bull. Mus. Comp. Zool., vol. 58, No. 4, 1914, pl. i).