246 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



AA. Pinnule ambulacra not plated. 



a. a pinnule on the third (epizygal) brachial. 



b. costals united by syzygy ; disk always more or less plated . . 



Zygometra {Antedon microdiscus Bell). 



bb. costals united by bifascial articulation ; disk naked, or with small, 



scattered, calcareous granules. 



c. lower pinnules stout and prismatic, subequal in length ; 



costals and lower brachials in close apposition, with sharply 



flattened sides. 



d. one of the lower pinnules somewhat enlarged; first 



two brachials not enlarged ; first pinnule as large as or 



larger than the second or third ; distal pinnules do not 



extend beyond tip of arm ; brachials long, triangular, 



or quadrate ; 10 arms 



Nanometra (Antedon minor A. H. Clark). 

 dd. lower pinnules about equal in size, but the first some- 

 what shorter than those following ; distal pinnules 

 extend beyond tip of aim ; brachials very short, ob- 

 long, or short-quadrate, the first two disproportionately 



large; 10 arms 



Tropiometra (Comatula carinata Lamarck). 

 cc. one or more of the lower pinnules elongated, slender, and 

 flagellate, cylindrical, or flattened. 

 d. the greatly elongated and flagellate lower pinnules are 

 composed of very numerous short and broad segments, 

 and are more or less serrate toward the tip ; costals 

 always well-separated and rounded ; centro-dorsal hem- 

 ispherical, with very numerous cirri, which are long with 

 numerous segments, long proximally, shorter distally, 

 where they are sharply carinate or bear low spines ; 

 terminal claw curved, moderate in length, or short, 

 always with an opposing spine ; middle and distal 



brachials triangular or quadrate ; 10 arms 



Heliometra (Alecto eschrichtii J. Muller). 

 dd. the first of the greatly elongated and flagellate lower 

 pinnules is composed of very numerous short and 

 broad segments ; but the others are composed of 

 greatly elongated smooth segments ; rays rounded, 

 well-separated ; centro-dorsal discoidal, bearing very 

 numerous cirri, which are long, with greatly elongated 

 smooth segments ; terminal claw long and nearly 

 straight, with no opposing spine ; middle and distal 



brachials oblong ; 10 arms 



Thysanometra (Antedon tenelloides A. H. Clark). 



ddd. all of the lower pinnules have elongated segments. 



e. first segment of the elongated lower pinnules 



always short ; costals and lower brachials 



usually rounded and free laterally, occasionally 



