164 ANNULOSA. 



bation of the thorax becomes very indistinct. This is likewise 

 the case, but to a less extent, in the genus Homalonotus. 



Fig. 108. Asaphiis Canadensis (Chap- 

 man), Lower Silurian. 



Fig. loa.Illefiius Barr- 

 iensis (Murchison), Lower 

 Silurian. 



The caudal shield or " pygidium " commonly called the 

 tail " is composed of a greater or less number of segments 

 anchylosed or amalgamated. Commonly, the pygidium is tri- 



Fig. i to. C-labella and pygidium of Diktloccpluilus inagnificus, Quebec Group 

 (Upper Cambrian). After Billings. 



lobed (fig. no), like the thorax, and consists of a central 

 elevated " axis '' and of a marginal " limb." The limb is 



