CYSTOIDEA. 291 



Silurian) of Britain, Cystideans are abundant fossils. In 

 the Upper Silurian (Niagara and Lower Helderberg) of North 

 America, and on the same horizon (Wenlock and Ludlow) of 

 Britain, Cystideans still occur, though their remains are not 

 so plentiful. Lastly, Cystideans certainly existed in the 

 Devonian period, the genus Strobilocystites being an undoubted 

 representative of this order in strata of this age in Iowa. 

 Some, however, of the so-called "Cystideans" of the Devonian 

 rocks appear to be probably referable to the Protozoa, and 

 others belong to the Blastoidea. With the close of the 

 Upper Silurian age, however, the order must have unques- 

 tionably greatly diminished, and we have no evidence of its 

 having survived the Devonian period, except in the aberrant 

 Codaster and Codonites of the Carboniferous. 



As regards their zoological affinities, the Cystoidea are 

 related to both the Crinoidea and the Blastoidea, From the 

 Crinoids they are separated by the, generally speaking, less 

 definite or unsym metrical development of the plates of the 

 perisome ; by the absence or rudimentary condition of the 

 arms ; by the usual possession of definitely arranged pores or 

 fissures in the test, which subserve a respiratory function ; 

 and by the fact that the reproductive organs appear to have 

 been always lodged in the interior of the calice, instead of 

 being situated under the integument of the arms or pinnulse. 

 To the Blastoids the Cystideans are more nearly related; but 

 in the former the internal cavities of the hydrospires and 

 pinnulae are directly united through the intervention of the 

 ambulacral pores (Billings), whereas in the latter they have 

 no connection with one another. Moreover, the recumbent 

 arms are directly imbedded in the substance of the test, 

 whereas in the Cystoids such structures, if present at all, lie 

 above and upon the plates of the test. 



The classification of the Cystoids is still in a very un- 

 satisfactory condition, largely in consequence of the fact that 

 the true structure of many types is yet very imperfectly 

 understood. There are, however, some fairly recognisable 

 groups, which may be briefly noticed, along with some of the 

 more prominent types belonging to each. In the first of 

 these groups, as typified by Cryptocrinus (fig. 174, B), the 



