THE UPPER SILURIAN PERIOD. 



125 



Eucalyptocrinus, Ichthyocrinus, M ariacrinus , Periechocrinus, 

 Glyptocrimis, Crotalocrlnus, and Edriocrinus. 



The tracks and burrows of Annelides are as abundant in 

 the Upper Silurian strata as in older deposits, and have just 

 as commonly been regarded as plants. The most abundant 

 forms are the cylindrical, twisted bodies (Planolites), which are 

 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 

 the filled-up burrows of marine worms resembling the living 

 Lobe-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 Lewisii. 



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 



Fig. 63. Planolites vulgaris, the filled-up burrows of a marine worm. 

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



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- 



