176 



HISTORICAL PALEONTOLOGY. 



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

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



Fig. 118. — A, Pentremiies pyrifo-nnis, side-view of the body ("calyx") ; B, The same 

 viewed from below, showing the arrangement of the plates ; C, Body of Pentretnites 

 conoidejts, viewed from above. Carboniferous. 



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 

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

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

 transversely-striated areas (fig. 118, C), each with a longitudi- 

 nal groove down its middle; and along each side of each of 



