The carapace which covers the =head= is known as the cephalic shield (Fig. 137, A, 1), and is commonly more or less semicircular in outline, but varies considerably in different genera. Only in a few cases, as in some species of _Agnostus_ (Fig. 146), is its length greater than its breadth. The axial part of the cephalic shield, called the “glabella” (Fig. 137, A, _a_), is usually more convex than the lateral parts (“cheeks” or “genae”), and is separated from them by longitudinal or axial furrows (_b_). The shape of the glabella varies greatly; it may be oblong, circular, semi-cylindrical, pyriform, spherical, etc. Its relative size likewise varies; thus in _Phacops cephalotes_ it expands in front and forms the larger part of the head, whilst in _Arethusina_ (Fig. 151, B) it is narrow and short, being only about one-half of the length of the head. [Illustration: FIG. 137.—_Calymene tuberculata_, Brünn. × 1. Silurian, Dudley. =A=, Dorsal surface: 1, head; 2, thorax; 3, pygidium or abdomen. _a_, Glabella; _b_, axial furrow; _c_, glabella-furrow; _d_, neck-furrow; _e_, fixed cheek; _f_, free cheek; _g_, facial suture; _h_, eye; _i_, genal angle; _k_, axis of thorax; _l_, pleura. =B=, Ventral surface of head (after Barrande): _a_, hypostome; _b_, doublure; _c_, _c′_, facial sutures; _d_, rostral suture; _e_, rostral plate. =C=, One segment of the thorax: _a_, ring of axis; _b_, groove; _c_, articular portion; _d_, axial furrow; _d-f_, pleura; _d-e_, internal part of pleura; _e-f_, external part of pleura; _e_, fulcrum; _g_, groove. =D=, Coiled specimen: _a_, glabella; _b_, eye; _c_, facial suture; _d_, pygidium; _e_, rostral suture; _f_, continuation of facial suture. ] The segmentation of the head is indicated by transverse furrows on the glabella (Fig. 137, A, _c_, _d_). In some cases these furrows extend quite across the glabella (Fig. 147), but commonly they are found on the sides only and divide the glabella into lateral lobes. Only the posterior or “neck-furrow” (Fig. 137, A, _d_) is continued on to the cheeks, and the segment which it limits anteriorly on the glabella[180] is known as the occipital or neck-ring. In front of the neck-furrow there may be three other furrows, so that altogether five cephalic segments are indicated by the furrows of the glabella. Commonly all the furrows are distinct in the primitive types; but in the more modified forms some, especially the anterior, become either reduced in size or obsolete. The actual number of furrows present consequently varies in different genera, and may even differ in different species of the same genus. In a few genera all the furrows are either indistinct or absent, as for example in _Ellipsocephalus_ (Fig. 150, B). In some cases four furrows are present in addition to the neck-furrow; this is due to the division of the anterior lobe of the glabella by fulcra which are developed for the attachment of muscles.