Clark] NEW GENERA OF RECENT FREE CRINOIDS 345 



for instance, the "Basicurva group" contains species both with and 

 without plated ambulacra ; the "Milberti group" contains two species 

 really belonging to the "Tenella group," and others only remotely 

 related to Antedon milberti, etc. The genera here proposed are 

 capable of exact and diagnostic definition, and occupy definite and 

 characteristic geographical areas, as well as bathymetrical and ther- 

 mal altitudes, which is the case with but one or two of the units 

 heretofore used in the classification of the finer divisions of the old 

 genus Antedon. 



No attempt is made in this paper to discuss the relationships of 

 the fossil Antedonidse and Actinometridae, a study of these forms 

 having been rendered impossible through lack of material ; I hope, 

 however, to be able to consider them later. In the list of species at 

 the end of each genus names of species which have not been described 

 accurately enough to admit of generic determination, or names re- 

 garded as synonyms, are omitted ; but a synonymy of all the species 

 has been worked out, and will probably be published in the near 

 future. 



Key to the Genera Described 



A. Pinnule ambulacra without covering plates. 



a. Costals united by syzygy ( i) Zygometra 



aa. Costals united by bifascial articulation. 



b. A pinnule on the 4th (epizygal) brachial. 



c. Lower pinnules stout and prismatic, subequal in length; costals 

 and lower brachials sharply "wall-sided." 

 d. First pinnule as large as or larger than the second or third ; 

 brachials long, triangular or quadrate ; first two bra- 

 chials not enlarged; always light yellow in color. 



(2) Nanometra 



dd. First pinnule smaller than those following; brachials very 



short, discoidal ; first two brachials much enlarged; 



deep purple or reddish brown in color, usually 



blotched with yellow (3) Tropiometra 



cc. Lower pinnules greatly elongated, slender, and flagellate, the 

 first with very numerous short and broad joints; 

 costals rounded, well separated. 

 </. All the greatly elongated lower pinnules are composed of 

 short and broad joints, and are more or less serr'ite 

 toward the tip; centrodorsal hemispherical with very 

 numerous cirri; cirri long, the proximal segment- 

 elongate, the distal short, sharply carinate or spiny 

 always with an opposing spine; terminal claw curved. 

 moderate in length or short; brachials quadrate or 



triangular ; yellow (4) H&liometra 



dd. All but the first of the elongated lower pinnules are com- 

 posed of greatly elongated joints and are smo t'i 



