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 



Fie. 60. Upper Silurian Star-fishes, i, Pa'asterina primirua, Lower Ludlow ; 2, 

 Palceaster Rut/went, Lower Ludlow ; 3, Palaocoma Colvini, Lower Ludlow. (After 

 Salter.) 



exquisitely-preserved examples of this group with which we 

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



Fig. 61. A, Protaster Sedffwickii, 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 



