THE UPPER SILURIAN PERIOD. 



121 



plates of these creatures, and some of them (such as the Wen- 

 lock Limestone of Dudley) have yielded perhaps the most 



Fig. 60. Upper Silurian Star-fishes, i, Palasterina primcrva, Lower Ludlow ; 2, 

 Palce'astcr Rutkveni, Lower Ludlow ; 3, PaUeocoma Colvini, Lower Ludlow. (After 

 Sailer.) 



exquisitely-preserved examples of this group with which we 

 are as yet acquainted. However varied in their forms, these 



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

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



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 innu- 

 merable calcareous joints or pieces, movably united with one 



