On the ventral surface of the head there is a flat rim around the margin (Fig. 137, B, _b_); this rim or “doublure” is the reflexed border of the cephalic shield. In many Trilobites its median part in front is cut off by sutures so as to form a separate plate (_e_); such is the case when the two facial sutures (_c_, _c′_) cut the anterior margin of the cephalic shield and are continued across the doublure, where they are joined by a transverse or rostral suture (_d_) just below the margin. When, however, as in _Phacops_ and _Remopleurides_, the two facial sutures unite on the dorsal surface, in front of the glabella, the median part of the doublure is not separated from the lateral parts, or from the dorsal part of the cephalic shield. [Illustration: FIG. 141.—=A=, Hypostome of _Bronteus polyactin_, Ang. showing maculae, × 4. =B=, Left macula of _Bronteus irradians_, Lindst. × 12. (After Lindström.) ] The “labrum” or “hypostome” is attached to the doublure in front (Fig. 137, B, _a_); it is commonly an oval or shield-shaped plate, but is occasionally nearly square. Its surface is sometimes divided into two or three areas by shallow transverse grooves (Fig. 141, A), Just behind the middle of the hypostome, or when transverse grooves are present either in or near the anterior groove, there are often found a pair of small patches or “maculae” which are more or less oval or elliptical in outline (Fig. 141). The maculae may be (1) surrounded by a raised border, or (2) in the form of pits, or (3) raised like tubercles. In some cases the entire surface of a macula is smooth and glossy; in others either the whole or a part is covered with granules, and in the latter case the granules may be limited to the internal third (Fig. 141, B) or to the central portion. Sections of a macula show that the granules are really globular lenses similar to those of the compound eyes on the dorsal surface of the head. Some of the maculae which are without lenses show no structure, but in others there is a spongy or irregularly polyhedric structure with prisms, resembling the marginal zone of the prismatic eyes of some genera. There seems no doubt that the maculae with lenses are visual organs, and those without are degenerate eyes. They occur in some genera which, according to Lindström, are without eyes on the dorsal surface. Maculae do not appear to be present in other Crustacea, but they have been compared with a median organ, found just in front of the hypostome in _Branchipus_.[189] Maculae, have so far been found in 136 species of Trilobites belonging to 39 genera ranging from Lower Cambrian to Carboniferous. A “metastoma” or lower lip plate (Fig. 142, _Ep_) is found just behind the hypostome in _Triarthrus_, but has not been noticed in any other genus. Between the hypostome and the metastoma lies the mouth.