Silurian; and if they do not become finally extinct here, they certainly survive the close of this period by but a very brief time.  By far the most important, however, of the Upper Silurian Echinodenns, are the Sea-lilies or Crinoids.  The limestones of this period are often largely composed of the fragmentary columns and detached plates of these creatures, and some of them (such as the Wenlock Limestone of Dudley) have yielded perhaps the most exquisitely-preserved examples of this group with which we are as yet acquainted.  However varied in their forms, these beautiful organisms consist of a globular, ovate, or pear-shaped body (the “calyx"), supported upon a longer or shorter jointed stem (or “column").  The body is covered externally with an armour of closely-fitting calcareous plates (fig. 62), and its upper surface is protected by similar but smaller plates more loosely connected by a leathery integument.  From the upper surface of the body, round its margin, springs a series of longer or shorter flexible processes, composed of innumerable calcareous joints or pieces, movably united with one another.  The arms are typically five in number; but they generally subdivide at least once, sometimes twice, and they are furnished with similar but more slender lateral branches or “pinnules,” thus giving rise to a crown of delicate feathery plumes.  The “column” is the stem by which the animal is attached permanently to the bottom of the sea; and it is composed of numerous separate plates, so jointed together that whilst the amount of movement between any two pieces must be very limited, the entire column acquires more or less flexibility, allowing the organism as a whole to wave backwards and forwards on its stalk.  Into the exquisite minutioe of structure by which the innumerable parts entering into the composition of a single Crinoid are adapted for their proper purposes in 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.