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 (Tubicola). Of 

 this nature are the tubes of Serpulites and Cornulites, and the 

 little spiral discs of Spirorbis Leivisii. 



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 Trilo- 

 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, Praetus, Lichas, Acidaspis, Illcenus, Caly- 

 me/ie, Homalonotus, and Phacops the last of these, one of the 



