no 



THE CRINOIDEA 



right and left correspond with the right and left of the observer. 

 To preserve this orientation when the crown is viewed from above, 

 the anal side must be nearest the observer (downwards in a 

 figure) ; when viewed from below, the anal side must be away 

 from the observer (upwards in a figure). Such is the rule followed 

 in the drawings illustrating this book, while in the various analyses 

 the anterior radius is always placed on the right of the figure. 

 Various modes of designating the radii have been attempted. To 

 extend to the Crinoidea Loven's Echinoid numeration, is to postu- 

 late an homology that is far from proven. The annexed table 

 compares with other systems the symbols here used : 



We have now run through the chief characters of a normal 

 crinoid of simple structure. Few are so simple as this, but 

 various modifications have occurred in the history of the class, 

 some perhaps only once, others at different geological periods in 

 races of diverse origin. Some of these, especially when of im- 

 portance for classification, must now be discussed. 



Some crinoids have a Dicyclic, others a Monocyclic Base (p. 

 99). The value of this in classification is disputed. Among 

 various early genera, placed by Wachsmuth & Springer in a 

 single family (Keteocrinidae), some are with, some without infra- 

 basals, having the angles of the stem-lumen respectively radial 

 and interradial. But the yet simpler genera, from which presum- 



