204 PALEONTOLOGY 



of the thorax and pygidium. Consequently in an enrolled 

 specimen only the head is seen on one surface, and only 

 the thorax and pygidium with the projecting brim and 

 genal spines on the other : in fact it is rather doubled-up 

 than rolled-up. 



The head within the brim consists of a highly raised 

 glabella widening forwards from about 3 mm. to about 

 5 mm., and smooth protuberant cheeks. The glabella 

 shows scarcely any sign of segmentation, and the cheeks 

 show no trace of eyes or facial suture. Trinucleus is thus 

 a blind trilobite, and has probably arrived at that con- 

 dition by degeneration consequent on complete adaptation 

 to the mud-grubbing life for which its shape is so well 

 suited. In certain species of Trinucleus (or closely allied 

 genera) there are traces of eyes in the young, and the 

 late Cambrian genus Orometopus, probably the ancestor 

 of Trinucleus, has well-developed eyes and opisthoparian 

 suture. With the loss of the eyes moulting became an 

 easier process and a facial suture could be dispensed 

 with, splitting taking place along the margin. The dis- 

 appearance of glabellar furrows is also a specialization, 

 and traces of them can be seen in some species of 

 Trinucleus (as T.favus). 



The thorax consists of six somites ; the axis is only 

 3 mm. wide, or one-fourth the total width. The pygidium 

 is short, obtusely triangular in shape, the axis tapering, 

 the number of somites about four. 



From what has been said in the chapter on Lamelli- 

 branchia, it will be understood that in seeking a natural 

 classification of trilobites it is essential to distinguish 



