This development can be followed in detail through the excellent figures of the larval stages of Sao hirsuta which BARRANDE has given in his magnificent work, vol. I, pl. 7. In the _first_ stage figured (figs. 1 d-e) the whole animal consists only of the head shield which is completely smooth, the glabella scarcely segmented, no facial ridge, no facial suture. In the _second_ stage, according to me (figs. 2-4 a), the glabella has become distinctly segmented, and the pygidium and partly the thorax have been added. In the _third_ stage, 4 c, d-9, one small narrow string exits from each side of the front of the glabella, making the first faint beginning of the facial ridge. They form a right angle with the glabella. In continued growth they become by degrees a little more curved (figs. 5 c, 6 b) and the lengthened ridge bends parallel to the outer lateral margin of the head (fig. 9 b). In fig. 9 d it has become so far complete that it reaches nearly back to the posterior cephalic margin, but is still of the same narrowness all along. In the _fourth_ stage, in a specimen (fig. 10 a) of 3 mm. in length, the facial suture makes its first appearance, setting in from the posterior margin of the head and meeting in its forward progress the posterior extremity of the facial ridge which now begins to swell out. It seems that both have a mutual influence on each other, the suture being deviated from a straight course[21] to take a bend outside the ridge, and the extremity of the ridge again at this contact to increase in size so as to form the elongated tubercle, often called palpebral lobe (fig. 14 b). This now augments in the same rate as the whole body. The characters of the four stages of the development of the larva consequently are: 1:o The archaic stage, only head shield with ridgelike glabella. 2:o The coming and growth of the pygidium and the complete segmentation of the glabella. 3:o The coming and growth of the facial ridge. 4:o The coming and development of the facial suture. [Footnote 21: In Trinucleus and Ampyx where there is no facial ridge, the suture has a straight direction along the lateral margins.]