360 GEOLOGY. 



resembled a flower in general aspect than an animal; and the familar 

 name "sea-lily" is not inappropriate. Clad throughout in calcareous 

 plates, the crinoids were excellent subjects of fossilization save that 

 after the binding tissues decayed, the constituent plates easily fell 

 apart and perfect specimens are rare. The stem, consisting of a series 

 of disks ("little grindstones") perforated by a central canal, and ren- 

 dered flexible by a minimum of integument, was disjointed with equal 

 ease. Limestone is sometimes largely made up of crinoidal fragments. 



The structure of the cystoids (Fig. 165, a-/) was very similar "to 

 that of the crinoids, but the body was unsymmetrical both in form 

 and in the arrangement of the plates. The outline of. the cystoid 

 was sometimes spheroidal or ovate, sometimes sac-like or club-shaped, 

 sometimes depressed and disk-shaped, with irregular departures from 

 all these forms. A part or all of the plates were perforated by small 

 pores connected with internal canals supposed to be concerned in 

 respiration, this being perhaps the most vital ground of distinction 

 from the crinoids, which do not usually show these. The arms were 

 few and irregular, and sometimes wanting; the stem usually short 

 and tapering, and often unattached. 



Very little can be said regarding the evolution of the cystoids, for 

 their forms are so heterogeneous and their functions so little known 

 that it is not clear what constituted progress, and besides, they were 

 so scantily represented in the preceding period, and so soon dropped 

 away after the Ordovician, that an insufficient interval for comparison 

 is afforded. The rise of the crinoids and the displacement of the cys- 

 toids by them is doubtless to be regarded as a phase of progress. The 

 two classes lived in comparative abundance through the Ordovician 

 period, and on into the Silurian, but by the close of the latter period 

 the cystoids had greatly declined, while the crinoids were in the height 

 of their abundance. 



The asteroids and ophiurians (Fig. 165, o and p) were not abundant 

 in the Ordovician, their chief development being reached much later 

 than that of the two preceding classes. The same is even more true 

 of the echinoids, which are barely represented in the strata of Europe. 



The development of the ccelenterates. — Corals were sparingly pres- 

 ent in the early part of the period, increased measurably in the middle 

 portion, and somewhat more markedly toward the close, but they 

 were not at any time a very prominent factor in the fauna. For the 



