22 4 



TRILOBITA 



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 l 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 



h 6 



FIG. 137. Ccdymene tuberculata, Briinu. x 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 ; /, free cheek ; g, facial suture ; 

 h, eye ; i, genal angle ; k, axis of thorax ; I, 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 ; /, 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 Ellipso- 

 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. 



