224 TRILOBITA CHAP. 
glabella into lateral lobes. Only the posterior or “ neck-furrow ” 
(Fig. 137, A, d) is continued on to the cheeks, and the seg- 
ment which it limits anteriorly on the glabella’ 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 
Fie. 137.—Calymene tuberculata, Brinn. x 1. Silurian, Dudley. A, Dorsal surface : 
1, head ; 2, thorax; 3, pygidium or abdomen. a, Glabella; 6, axial furrow ; ¢, 
glabella-furrow ; d@, neck-furrow ; e, fixed cheek ; 7, free cheek ; g, facial suture ; 
h, eye; 7, genal angle; x, axis of thorax ; 7, pleura. B, Ventral surface of head 
(after Barrande): a, hypostome; 6, doublure; c¢,c’, facial sutures; d, rostral 
suture; ¢, rostral plate. ©, One segment of the thorax: a, ring of axis; 3, 
groove ; ¢, articular portion ; d, axial furrow ; d-f, pleura; d-e, internal part of 
pleura ; e-f, external part of pleura; e, fulerum; g, groove. D, Coiled specimen : 
a, glabella ; 6, eye ; c, facial suture; d, pygidium ; e, rostral suture ; 7, continua- 
tion of facial suture. 
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 E/lipso- 
cephalus (Fig. 150, B). In some cases four furrows are present 
in addition to the neck-furrow ; this is due to the division of the 
1 On the cheek the furrow represents a pleural groove, and does not form the 
limit of the posterior cephalic segment. 
