It would naturally be expected that the young of the Cambrian trilobites should be more primitive than the young of species from later formations, and Beecher (1895 C) has shown that this is the case. He had reference, however, chiefly to the eyes, free cheeks, and spines, and by comparison of ontogeny and phylogeny, demonstrated the greater simplicity of the protaspis which lacked these organs. It remains to inquire which among the other characteristics are most fundamental.
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).
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.
From the foregoing brief description it appears that the pygidium of the protaspis varies in different groups from as little as 15 per cent of the total length in the Mesonacidæ to as much as 50 per cent in the Agnostidæ; that the axial lobe varies from as little as 14 per cent of the total width in one Ptychoparia to as much as 50 per cent in Phalacroma nudum; that the glabella reaches the anterior margin in the Olenidæ, Solenopleuridæ, and Phalacroma bibullatum, while there is a brim in front of it in the Olenellidæ, Paradoxidæ, and three of the species of the Agnostidæ. The decision as to which of these conditions are primitive may be settled quite satisfactorily by study of the ontogeny of the various species.