THE CARBONIFEROUS PERIOD. 179 



(fig. 117), Poteriocrinus, Zeacrinus, and Forbesiocrinus. Closely 

 allied to the Crinoids, or forming a kind of transition between 

 these and the Cystideans, is the little group of the " Pentre- 

 mites, " or Blastoids, (fig. 118). This group is first known to 

 have commenced its existence in the Upper Silurian, and it 

 increased considerably in numbers in the Devonian ; but it 

 was in the seas of the Carboniferous period that it attained its 

 maximum, and no certain representative of the family has been 

 detected in any later deposits. The " Pentremites " resemble 

 the Crinoids in having a cup-shaped body (fig. 118, A) enclosed 

 by closely-fitting calcareous plates, and supported on a short 

 stem or " column, " composed of numerous calcareous pieces 

 flexibly articulated together. They differ from the Crinoids, 

 however, in the fact that the upper surface of the body does 

 not support the crown of branched feathery " arms, " which are 

 so characteristic of the latter. On the contrary, the summit of 

 the cup is closed up in the fashion of a flower-bud, whence the 



Fig. 117. Platycrinus tricontadactylus, Lower Carboniferous. The left-hand 



figure shows the calyx, arms, and upper ,'part of the stem ; and the figure next this 



shows the surface of one of the joints of the column. The right-hand figure shows the 

 proboscis. (After M'Coy.) 



technical name of Blastoidea applied to the group (Gr. blastos, 

 a bud; eidos, form). From the top of the cup radiate five broad. 



