WOOD, CR/NOID ARMS IN STUDIES OF I'HYLOGENY gj 



In arrangement of the plates and surface ornament, this species does not 

 differ essentially from C. proboscidalis, there being the usual number of plates, 

 five basals, five each of radials, first and second costals and ten axillary dis- 

 tichals each of which bears two arms, making four to the ray. The arms of 

 the two species are similar in form, being cylindrical at the base, strongly 

 flattened laterally throughout the greater portion of their length, tapering 

 and becoming more nearly cylindrical near the tips. The most characteristic 

 difference is in the ornament of the arms, for while the arm plates of C. 

 proboscidalis bear a single transverse ridge throughout, the arms of G. baccatus 

 have this simple ridge only near the base of the arm. At a distance from the 

 base varying from 6 to 20 millimeters in different specimens and varying to 

 some extent in different arms of the same specimen, the transverse ridge is 

 broken up into a row of small nodes of which there are five or six on each 

 plate at the greatest diameter of the arm. 



The column was not preserved with any of the specimens found. 



Horizon and locality : Lower Burlington limestone, Burlington, Iowa. No. 

 558, Museum of Comparative Zoology collection. 



Cactocrinus platybrachiatus, sp. nov. 

 Plate I, fig. 2 ; plate III, figs. 1, 2, 2a, 2b 



The basals of the only specimen representing this species are not preserved. 

 Of the plates above the radials, only those of two rays and one interradial 

 area are preserved. Their arrangement seems to be the same as that usual in 

 the genus for five or six armed rays ; that is, in one of the half-rays present, 

 the axillary distichal is followed, without intervening plates, by an axillary 

 palmar which gives rise to two arms. The palmar resting on the other axillary 

 face of the distichal bears one arm giving three arms to this half ray. The 

 same arrangement is seen in another half ray, but whether there were two or 

 three arms in the other half of the same ray cannot be determined. There is 

 one small interdistichal, and the formula for the interbrachials is 1, 2, 2, 1. 



The ornament on the calyx is essentially the same as that of Cactocrinus 

 proboscidalis, but since there are more plates due to the greater number of 

 arms, and a ridge crosses each suture line between the plates, the costse ap- 

 pear more crowded than on the latter species. The node at the center of each 

 plate is also less prominent. 



The arms are cylindrical near the base but expand rapidly in the median 

 portion. They are flattened laterally but less strongly so than the arms of 

 Cactocrinus baccatus. The form of the arms changes gradually, until in the 

 upper portion they are flattened dorso-ventrally. Their tips are incurved 

 toward the anal tube and buried in the matrix. The biserial condition is at- 

 tained early, only one or two plates at the base passing entirely across the 

 arm. The surface of the arm is smooth at the base. A little later a transverse 

 ridge appears which soon breaks uj) into a row of nodes like those of Cacto- 

 crinus baccatus. This type of ornament persists for the greater portion of the 

 length of the arm. but by the time the dorso-ventral flattening is established, 

 some of the lateral nodes become confluent, reducing their number until there 

 are but three on each plate, and on some of the latest plates visible, there are 



