THE UPPER SILURIAN PERIOD. 123 
so frequently found on the surfaces of sandy beds, and which 
have been described as the stems of sea-weeds. ‘These fossils 
(fig. 63), however, can be nothing more, in most cases, than 
Fig. 63.—Planolites vulgaris, the filled-up burrows of a marine worm. 
Upper Silurian (Clinton Group), Canada. (Original.) 
the filled-up burrows of marine worms resembling the living 
Lob-worms. There are also various remains which belong to 
the group of the tube-inhabiting Annelides (Zudicola). Of 
this nature are the tubes of Serpudlites and Cornulites, and the 
little spiral discs of Spzvorbis Lewirsit. . 
Amongst the Articulates, we still meet only with the remains 
of Crustaceans. Besides the little bivalved Ostracoda—which 
here are occasionally found of the size of beans—and various 
Phyllopods of different kinds, we have an abundance of Z7z/o- 
bites. These last-mentioned ancient types, however, are now 
beginning to show signs of decadence ; and though still indi- 
vidually numerous, there is a great diminution in the number 
of generic types. Many of the old genera, which flourished 
so abundantly in Lower Silurian seas, have now died out; 
and the group is represented chiefly by species of Cheirurus, 
Encrinurus, Harpes, Proetus, Lichas, Acidaspis, Ilenus, Caly- 
mene, Homalonotus, and Phacops—the last of these, one of the 
