18 IOWA STUDIES IN NATURAL HISTORY 



short spines; cirri short and stout, 

 with short subequal segments most of 

 which bear high dorsal spines, the 

 more proximal with a transversely 

 elongate chisel-like edge 



Analcidomelra 



j2 first pinnule much elongated, but not 

 especially stout; no true dorsal spines 

 on the cirrus segments 

 ki centrodorsal rounded conical, al- 

 most entirely covered with from 

 40 to 60 cirrus sockets; cirri long, 

 rather slender, composed of from 

 25 to 30 segments; proximal 

 brachials with the distal edge ab- 

 ruptly everted and conspicuously 

 spinous; first pinnule more slen- 

 der than those following; division 

 series and lower brachials in close 

 lateral apposition and flattened 

 against their neighbors 



TricJiometra 



k2 centrodorsal very low, flattened 

 hemispherical, with less than 35 

 cirrus sockets; cirri short, rather 

 stout, with not more than 15 seg- 

 ments; no eversion of the distal 

 borders of the earlier brachials; 

 first pinnule stouter than those 

 succeeding; division series and 

 arm bases narrow and widely 

 separated; a cluster of perisomic 

 interradial plates in each inter- 

 radial angle 



Antedon 



d2 the ambulacral grooves on the pinnules and arms are bor- 

 dered by two rows of minute plates easily seen in dried 

 specimens of which those of the inner row can be closed 

 down over them 



el cirri short and stout, strongly curved, with not more 

 than 20 segments none of which bear dorsal spines; 

 centrodorsal very broad, more or less columnar or broad 

 truncated conical, the cirrus sockets closely crowded and 

 irregularly arranged; first two pinnules similar, slender. 



