THE UPPER SILURIAN PERIOD. 1 23 



SO frequently found on the surfaces of sandy beds, and which 

 have been described as the stems of sea-weeds. These fossils 

 (fig. 6$), however, can be nothing more, in most cases, than 



Fig. 63. — Planolites znilgaris, the fllled-up burrows of a marine worm. 

 Upper Silurian (Clinton Group), Canada. (Original.) 



the fiUed-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 {Tubicold). Of 

 this nature are the tubes of Serpulites and Cornidites^ and the 

 little spiral discs of Spu'orbis Lewisii. 



Amongst the Ai'ticulates^ 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, Jlarpes, F?-oetus, Lichas, Acidaspis, incB?ius, Caly- 

 mene, Hoinalonotus^ and Phacops — the last of these, one of the 



