14 ANNALS NEW YORK ACADEMY OF SCIENCES 



Cactocrinus limabrachiatus (Hall) 

 Plate V, figs. .*{, 4. 4(/, ih 



1861. Actinocritius limabrachialux H.ill, 1 ><>s(iii>ii«>n of New Species of Crinoi- 



dea, Preliminary notice, p. 2. 

 1897. CactocrinuH Uvinbritchiatux Wuchsmuth and Springer. North American 



C'rinoidea Camerata. p. <!0S. j)!. ,-),s, li«s. 9, 10a, 10^. 



The arrangement of ])]ates in (he calyx of this species is the same as 

 that already described for species having six arms to the ray. The sur- 

 face is highly ornamented with strong nodes and carinre, which, on the 

 larger calyces, are of two series. The carinji? leading to the arms are 

 much stronger than the others. 



The arms of each ray are grou[)e(l together, suggesting the arrange- 

 ment in Adinocrinus. although the spaces between the rays are still nar- 

 row. The arms are long and slender, cylindrical at the base, but they 

 soon become flattened dorso-ventrally. They are slightly expanded at a 

 distance of about half their length from the base and taper very gradually 

 to the tips. The biserial condition is reached late in the development of 

 the arm, there being from four to seven plates at the base which pass 

 entirely across its diameter. At a distance of from 5 to 8 millinu'ters 

 from the base, varying on different arms, each plate is ornamented by a 

 projecting transverse ridge near its upper margin, and the surface is (;ov- 

 ered by strong vertical corrugations. This type of ornament persists to 

 the tip of the arm, while its form changes from cylindrical at the base to 

 strongly flattened above, as shown by the transverse sections, Plate V, 

 figs. 4, 4a, 46. 



Horizon and locality : Lower P.urlington, liurlingtou, Iowa. No. 528, Mu- 

 seum of Comparative Zoology collection. 



Comparison of tite Preceding Three Species 



A comparison of Cactocrinus itiullibiuchmtus and C. ccelatus var. 

 spinotentaculus shows the relationship between the two to be so close that 

 they might be considered the same species, were it not that in a large 

 series of specimens, C. inultihrachi-atiis shows all the characters of an 

 adult individual, while it is only about half the size of C ca'Iatus var. 

 spinoientaculus. The proportionally much higher calyx of the latter is 

 also a distinctive feature. The arms of the two s]>ecies pass through the 

 same structural changes, which are iiiter]iroted as stages of development 

 and are closely similar, exccjpt that in (\ ccrJatita vm'. spinotentaculus, 

 they are biserial nearer the base and are larger. 



