the economy of the animal, it is impossible to enter here. No period, as before said, has yielded examples of greater beauty than the Upper Silurian, the principal genera represented being _Cyathocrinus, Platycrinus, Marsupiocrinus, Taxocrinus, Eucalyptocrinus, Ichthyocrinus, Mariacrinus, Periechocrinus, Glyptocrinus, Crotalocrinus_, and _Edriocrinus_. [Illustration: Fig 62.--Upper Silurian Crinoids. a, Calyx and arms of _Eucalyptocrinus polydactylus_, Wenlock Limestone; b, _Ichthyocrinus loevis_, Niagara Limestone, America; c, _Taxocrinus tuberculatus_, Wenlock Limestone. (After M'Coy and Hall.)] [Illustration: Fig. 63.--_Planolites vulgaris_, the filled-up burrows of a marine worm. Upper Silurian (Clinton Group), Canada. (Original.)] 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 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 _Cornultites_, and the little spiral discs of _Spirorbis Lewisii_. [Illustration: Fig. 64.--Upper Silurian Trilobites. a, _Cheirurus bimucronatus_, Wenlock and Caradoc; b, _Phacops longicaudatus_, Wenlock, Britain, and America; c, _Phacops Downingioe_, Wenlock and Ludlow; d, _Harpes ungula_, Upper Silurian, Bohemia. (After Salter and Barrande.)]