194 PAL/EONTOLOGY 



area, of which about 1-5 mm. is visible in an enrolled 

 specimen, but which almost disappears in a straight one. 

 The pleura vary in width from about 12 mm. in the first 

 somite to 10 mm. in the last; about 4 mm. from the axis 

 each bears a prominent little forward projection, fitting into 

 a socket in the somite in front, and serving as a pivot. In 

 enrolled specimens the part of the pleuron external to 

 this pivot is largely concealed beneath the somite in front. 

 A well-marked groove divides each pleuron into a smaller 

 anterior and larger posterior portion. 



The head shows the same trilobed character as the 

 thorax : the central part is called the glabella, the lateral, 

 the cheeks or gents. Evidences of segmentation are 

 shown chiefly by the glabella ; there is, however, one 

 well-marked cross-furrow (neck or occipital furrow) near 

 the posterior margin, marking off both in glabella and 

 genae a posterior somite which closely resembles a 

 thoracic somite, but is immovably fixed to the rest of the 

 head-shield. The other signs of segmentation are three 

 pairs of grooves (segmental furrows) in the glabella, more 

 or less transversely placed, but not crossing the centre 

 of the glabella, which is quite smooth. The most anterior 

 of. these is very slight, the second longer or deeper, the 

 third still longer and deeper. The two latter bend 

 obliquely backward, so as to mark off rounded lobes 

 joined to the rest of the glabella by a constricted base. 

 These three furrows and the occipital furrow indicate 

 that the head is composed of five united somites With 

 the disappearance of flexibility in the head, why have 

 traces of the grooves between the somites been retained ? 



