SYSTEMATIC PART. 



169 



throw new light upon important questions, and require the undoing of 

 much previous work ; — as was illustrated in the case of the disk of the 

 Ichthyocrinidae, and by de Loriol's discovery of small infrabasals in two 

 species of MUlericrinus, which made important changes in the classification 

 imperative. 



DEFINITION OF THE CRINOIDEA AND THEIE PRIMAEY DIVISIONS. 



Class CEINOIDEA. 



Echinoderms which during part or all of their life were attached to other 

 objects, either by means of a stem or directly by the abactinal side of the 

 calyx. The visceral mass enclosed by a limestone test or calyx, constructed 

 of plates symmetrically arranged, and giving off well defined, free arms 

 from the sides. Mouth directed upwards. 



I. 



Crinoids in which the arms are free above the radials. The top 



joint the youngest in the stem Order Inadunata. 



A. Disk composed of orals only. Ambulacra subtegminal Suborder Larviformia. 



B. Posterior side of the disk extended into a sac. Am- 



bulacra supra-tegminal Suborder Fistulata. 



II. 



Crinoids in which the lower brachials take part in the dorsal cup. 

 All plates of the calyx united by close suture. Mouth and 

 food-grooves closed. The top joint the youngest in the stem. 



Order Camera t a. 



III. 



Crinoids in which the lower brachials are incorporated into the 

 calyx either by lateral union with each other, or by supple- 

 mentary plates, or a calcareous skin. All plates from the 

 radials up movable. Mouth and food-grooves exposed. The 

 top stem joint fused with the infrabasals, and not the youngest 

 joint of the stem Order Articulata. 



A. Arms non-pinnulate Suborder Impinnata. 



B. Arms pinnulate Suborder Pinnata. 



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