_Sao hirsuta_ is a species found in the Cambrian, the development of which was fully described by Barrande. Its earliest protaspis stage (Fig. 143, A) is circular in outline; the glabella expands in front and reaches the anterior margin; the pygidial region is not distinctly separated from the cephalic region; segmentation is indicated in the former, and the neck-ring is present in the latter; the eye-line is seen on each side of the glabella near the anterior margin. In a later stage (Fig. 143, C) the segmentation of the glabella becomes more distinct, indicating the existence of five cephalic segments, and the facial suture appears near the margin limiting a very narrow free cheek. Subsequently (Fig. 143, D-F) the thoracic segments develop, and increase in number until the adult stage (G) is reached; also the eyes appear at the margin of the cephalic shield, and gradually move inwards, and the glabella becomes narrower and rounded in front, and ceases to reach the anterior margin. In this species the eye-line is present in the adult. [Illustration: FIG. 144.—_Triarthrus becki_, Green. Ordovician. =A=, =B=, Two successive stages of the protaspis, × 45. (After Beecher.) ] In the protaspis of _Triarthrus_ (Fig. 144), found in the Ordovician, the glabella does not reach the front margin nor expand in front as it does in _Sao_; an eye-line is present, but disappears before the adult stage is reached. [Illustration: FIG. 145.—Larval stages of Trilobites. =A-D=, _Dalmanites socialis_, Barr. Ordovician, Bohemia. The small figures below show the natural size of each specimen. (After Barrande.) =E=, _Mesonacis asaphoides_, Emmons, × 10. Lower Cambrian, North America. (After Walcott.) =F=, _Acidaspis tuberculata_, Conrad, × 20. Lower Helderberg Group (Lower Devonian or Upper Silurian), Albany County. (After Beecher.) ] _Dalmanites_ (Fig. 151, C) is a more advanced type than _Sao_ and _Triarthrus_, and is found in later deposits. In the earliest stage (Fig. 145, A) the head and pygidium are quite distinct, and there is no eye-line present at this or any stage in development, but large ovoid eyes are found on the front margin, and have their long axes placed transversely to the axis of the body; the glabella is strongly segmented and is rounded in front. In later stages (C, D) the pygidium increases in size relatively, and the thoracic segments are successively introduced; the facial sutures and free cheeks appear on the dorsal surface, and as the free cheeks grow the eyes move inwards and backwards, and gradually swing round until their long axes become parallel with the axis of the body. The larval form of _Acidaspis_ (Fig. 145, F) is of interest since even in the earliest stage it shows the spiny character which forms such a striking feature of the adult (Fig. 151, F).