6 AAWALS NEW YORK ACADEMY OF SCIENCES 



ouly two, a shorter node uear the median line of the arm and an elongated 

 one placed laterally. 



The column is unknown. 



IToKizoN AND locality: Lower I'.urlinKton limestone, P.urlington. Iowa. No. 

 HGS, Museum of Coiui>arative Zoillogy collection. 



Cactocrinus platijhrachialus is distinguishccf from C. baccatus by the 

 greater number of arms, the dorso- ventral flattening of the arms and the 

 confluence of their surface nodes in the upper part of the arm, probably 

 representing a late stage of develo])nicnt. It is distinguished from C. 

 reliculalus by tlie many nodes on the ai'ms at the maximum differentia- 

 tion in structure, rei)resenting the adult stage, the less strongly flattened 

 arms and the absence ot lateral spines on the arms near their tips. 



Cactocrinus reticulatus (Hall) 

 Plate III, ligs. :;, 4, 4a, 4b, 4c 



1861. Actinocrinus rrticultitus Ilali, Description of New Species of ("rinoidea, 



I'reliminary notice, p. 2. 

 1897. Cactocrinus rcticuUitux Wachsmuth and Springer, North American 



('rinoidea C^amerata. p. (tcri. pi. ."s. tigs. 2(;, 2h. 



The arrangement of the calyx plates in this species is somewhat vari- 

 able, owing to the fact that it has sometimes five and sometimes six arms 

 to the ray. Of six specimens selected at random, three had 28 arms, one 

 27, one 24 and one 22 arms. When there are five arms to the ray, it is 

 always one of the median palmars which is axillary and bears two arms, 

 while with six arms to the ray, both median palmars become axillary and 

 the lateral pahnars bear a single arm. On the calyx of large individuals, 

 the prinuiry costae are sometimes bordered by a second series producing 

 a smaller triangle within a larger one. The nodes at the centers of the 

 plates are inconspicuous or sometimes absent. 



In form, the arms are cyliniirical at the base, but soon their plates are 

 elongated as in Cactocrinus prubo.scidalis (see Plate II, fig. 1). A little 

 later, the plati^s are curved, forming an arm equal in lateral and dorso- 

 ventral diameters. At a slightly higher point, the arms are flattened 

 dorso-ventrally, and the latest of the exposed plates are strongly flattened 

 in the same direction. The ornament begins on the early plates of the 

 arm as a strong transverse ridge or elongate node near the lateral margin, 

 and a few plates later, a new node appears near the median line of the 

 arm. As growth continues, these two nodes appear on successive plates 

 nearer and nearer to the lateral margin, and when they have receded far 

 enough to leave a jilain space near the iiKMJiaii line, a new node appears in 



