278 HISTORICAL GEOLOGY. 



as is shown by the fact that some free echinoderms have 

 stems in the early stages of life, and afterward throw 

 them off and become free. 



Description of a Crinoid. A crinoid has a pear- 

 shaped body, containing the viscera, set upon a jointed 



FKJ. 182. FIG. 183. FIG. 184. 



FIGS. 182-184. Silurian crinoids : 182. Heterocrinus simplex. (After Meek.) 183. 

 Pleurocystitis squamosus. 184. Lepadocrinus Gebhardii. 



stem, with mouth on the top of the pear, sometimes sur- 

 rounded by many plumose arms (Fig. 182), sometimes 

 with few simple arms (Fig. 183), sometimes with no arms 

 at all (Fig. 184). 



Range in Time. We have said that stemmed echino- 

 derms or crinoids continue from earliest times until noiv 

 (though the species and genera change repeatedly), but 

 in diminishing numbers. The free echinoderms, on the 

 contrary, have been constantly increasing. If, then, A B 

 (Fig. 185) represent the course of geological time, and 

 the parallelogram the equal abundance of echinoderms 

 throughout, then the shaded portion below the diagonal 

 would, in a general way, represent the constantly decreas- 

 ing stemmed, and the unshaded space above the diagonal 



