THE UPPER SILURIAN PERIOD. 



123 



of this period by but a very brief time. By far the most im- 

 portant, however, of the Upper Silurian Echinoderms," are the 



Fig. 60. Upper Silurian Star-fishes. 1, Palasterina primceva- Lower Ludlow ; 2, 

 Palceaster Ruthveni Lower Ludlow; 3, Palceocoma Colvini, Lower Ludlow. (After 

 Salter.) 



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 Wen- 



rig. 61. A, Protaster Sedgwickii, showing the disc and bases of the arms ; B, Por- 

 tion of an arm, greatly enlarged. Lower Ludlow. (After Salter. ) 



lock 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 consists 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 



