WOOD, CRINOID ARMf^ IN t^TUD/ES OF f'HYLOOENY 13 



number is frequently present. The post-palmars, palmars and axillary 

 distichals rest one upon another without intervening plates, following the 

 usual plan in the genus Cactocrinus. The nodes at the centers of the 

 plates are not prominent, and the costaa are simple, except on the lower 

 half of the radials, where there are sometimes two or three passing to the 

 basals. 



The arms are long and slender, tapering very gradually to the tips and 

 but slightly incurved. They are cylindrical for a distance of about twenty 

 to thirty plates from the base and then become somewhat flattened dorso- 

 ventrally, developing an obtuse angulation along the lateral margin. At 

 about half their length, the arms are somewhat expanded laterally, and 

 at this point or a little higher, tliey develop a narrow transverse ridge 

 close to the upper margin of each plate. These ridges give the arm an 

 appearance of being serrated along its lateral margin with each plate 

 slightly inset above its predecessor. The arm of C. multibrachiatus is 

 well represented by the figures of C. ccelatus spinotentacului^ on Plate V, 

 fig. 1, except that it is all on a smaller scale. 



Horizon and locality : Lower Burlington. Burlington, Iowa. No. 548, Mu- 

 seum of Comparative Zoolog.v collection. 



Cactocrinus ccelatus var. spinotentaculus (Hall) 



Plate V, figs. 1, 2, 2a 



1860. Actinocrinus spinotentaculus Hall, Suppl. (Jeol. Rept. Iowa, p. 86. 

 1897. Cactocrinus cnclatus var. spinotentaculus Wachsmuth and Springer, North 

 American Crinoidea Camerata, p. 619, pi. 59, fig. 10. 



This species is closely similar to the preceding, except that it is much 

 lai'ger and the calyx is proportionally higher. The proportion of height 

 to width in C. ccelatus var. spinotentaculus is about 1: IV7 as compared 

 with 1 : 13/2 in C*. multihrachiatus. 



In arrangement of plates and surface ornament, the two species are 

 the same. The arms are eight to the ray and so similar to those of C. 

 multihrachiatus that the same drawing serves to represent the character- 

 istics of both, keeping in mind the fact that the present species is more 

 than twice the size of C. multihrachiatus. and the corrugations of the 

 surface, in common with other features, are much coarser. 



Horizon and locality: Lower Burlington. Burlington. Iowa. No. 552, Mu- 

 seum of Comparative Zoology collection. 



