WOOD, CRINOID ARMS IN STUDIES OF I'HYLOGENY 3 



Description of Cactocrinus and Six Species of the Genus 



Cactocrinus Wachsmuth and Springer 



1897. Cactocrinus Wachsmuth and Springer, North American Crinoidea 

 Camerata, p. 600. 



The species included in the genus Cactocrinus were, with few excep- 

 tions, originally descrihed under the genus Actinocrinus. and for the 

 present purpose, the genus is sufficiently defined by enumerating the fea- 

 tures by which it is distinguished from' Actinocrinus. The most notice- 

 able of such distinguishing characters seems to be the arrangement of the 

 arms, which form a more or less continuous row around the calyx ; that is, 

 the interbrachial and interambulacral plates do not meet between the 

 arms as is the case in Actinocrinus. Another distinguishing feature is 

 that, in species having more than foiu- arms to the ray, the third bifurca- 

 tion takes place on the second or third plate above the distichals in Acti- 

 nocHnus. while the axillary palmar immediately follows tlie axillary 

 distichal in Cactocrinus. The two genera are also said to differ in the 

 structure of the pinnules, but the spines on the proximal pinnules of 

 Cactocrinus are represented also on many species of Actinocrinus. as well 

 as on other genera as far removed as Dorycrinus and Eretinocrinus. 

 Hence this cannot be used as a distinctive feature. 



The type of the genus is Cactocrinus prohoscidalis (Hall). 



Cactocrinus prohoscidalis (Hall) 

 Plate I, fig. 1 ; plate II, figs. 1, 2, 2a, 26 



1858. ActinocrinuH prohoscidalis Hall, Rept. Geol. Surv. Iowa, Vol. 1, pt. 2, 



p. .^)84, pi. 10, fig. 13. 

 1897. Cactocrinus prohoscidalis Wachsmuth and Springer, North American 



Crinoidea Camerata, p. 601, pi. 58, figs. .3, 4, 5, 6, 7 a-d. 



The calyx of this species has a width slightly greater than its height. 

 There are, as usual in the genus, three basals, five radials and five each 

 of first ahd second costals. The axillary distichals rest directly upon the 

 second costals, and each of these gives rise to two arms, making four to 

 the ray. The surface of the calyx is ornamented by a node at the center 

 of each plate from which simple carinas radiate across the margins and 

 become continuous with the caringe from adjacent plates. The entire 

 surface of both cup and arms is covered with extremely fine granules. 



The arms are long, three or four times the height of the calyx. Be- 

 yond the first palmars, from one to three plates pass entirely across the 

 diameter of the arm, hence the biserial condition is attained early in this 



