12 A.\.\ALS M:\\ YORK ACADLAll OF SCIENCES 



no specimen which seems to fulfil all the requirements for such an an-; 

 cestor. We may, however, reasonably infer what were some of its char- 

 acteristics. The arrangement of the calyx plates was probably the same 

 as that of C. proboscid^lis, and the plates were nodose, either with or 

 without connecting carina). The arms were cylindrical, smooth, tapering 

 at the tips and the biserial condition was attained late, i. e., more than 

 two or three plates passed entirely across the diameter of the arm. Such 

 an ancestor would be expected to occur in strata older than those con- 

 taining C proboscidalis, and we should naturally look for it in the Kin- 

 derhook, but the species of Cactocrinus recorded from tlie Kinderhook, 

 C. nodohracliiatus, C. ornaiii^simus and C. arnoldi, have ornamented 

 arms of a type quite different from those of C. proboscidalis or any of its 

 descendants. This indicates that the ancestor of Cactocrinus must have 

 lived at a period considerably earlier than the Kinderhook. 



Cactocrinus thetis, of the Lower Burlington, has arms which in form 

 and surface are like those of the hypothetical ancestor of C. proboscidalis, 

 but they are biserial almost from their point of origin, and there are six 

 arms to the ray, while C. proboscidalis has but four. C. thetis was prob- 

 ably descended from the same ancestor as C. proboscidalis, but while the 

 latter has developed in the direction of surface ornament and modifica- 

 tion in the form of the arms, C. thetis has been retarded in surface orna- 

 ment and has advanced in the direction of number of arms and in the 

 early attainment of the biserial condition. The two species represent 

 divergent lines of evolution. 



Another species of Cnctocrintis whicli probably occupies a relation to 

 C. proboscidalis similar to that of C. thetis is C. thalia. The latter spe- 

 cies has long, slender, cylindrical, smooth arms, only four to the ray, but 

 it does not seem to be an ancestor of C. proboscidalis, since it is a larger 

 species with longer and more slender arms which become biserial at an 

 earlv stage. 



"{s"- 



Cactociunus MUL'i'ii;uA(iiiATi;s Si';i!ii:s 



Cactocrinus multibrachiatus (ITall) 



1858. Actinocnnus multibrachiatus Hall, Rept. Geol. Surv. Iowa, p. 580, pi. 10, 



fig. 10. 

 1897. Cactocrinus multihrnchiatns Wadisinuth and Springer, North American 



Crinoidea Camerata, p. 617, pi. ,56, tigs. 6, 7 ; pi. 58, fig. 8. 



The calyx of this species resembles that of C. proboscidalis except in 

 the greater number of plates necessary for the support of eight arms to 

 the ray, this being the normal number for the species, although a smaller 



