WOOD, CRIXOID AKM)S IN STUDIES OF PHYLOGENY H 



final stage in the evolution of the arm of C. den I leu la I us is more extreme 

 than that of C. retkulaius in that the arms are more strongly flattened, 

 the spines are longer and there is an alternation in the size of both spines 

 and nodes. The lateral movement of surface features is further illus- 

 trated in this species by the fact that on the latest plates observable, the 

 line of notles last introduced does not remain near the median line as on 

 C. reticulatus but has receded to some distance from it. 



This series of five species of crinoids appears to constitute an excellent 

 illustration of the principle of recapitulation, each member repeating the 

 life history of its ancestor until, in the later members, early stages are 

 crowded out of the ontogeny to be replaced by characters of later origin. 

 Cactocrinus opusculus, the sixth of the species described above, bears 

 a strong general resemblance to C. reticulatus, but it has always six arms 

 to the ray and consequently more plates in the calyx. The arms at the 

 latest stage observable are closely similar to those of C. reticulatus at the 

 same stage, but they have arrived at this condition along a different path 

 from that traversed by the latter species, as shown by a comparison of the 

 figures on Plate IV, fig. 3, with those on Plate III, fig. 3. The early stages 

 lack the strong lateral node seen on C. reticulatus, and there is no indica- 

 tion in the ontogeny of the species that it has passed through the stage 

 with rows of nodes present on C. haccatus and C. platyhi-achiatus. C. 

 opusculus seems to have been descended from C. prohoscidalis but as a 

 lateral branch, following a different line of evolution from that of the 

 C reticulatus series. The resemblance between the final stages in the 

 arms of C. opusculus and C. reticulatus may be considered a case of 

 parallelism. 



Another line of evolution from C. prohoscidalis, divergent from that 

 of the reticuJatus series, is represented by C. clarus. This species closely 

 resembles C. prohoscidalis in the calyx and in the strong lateral flatten- 

 ing of the arms, but it is a much larger species and has five or six arms 

 to the ray. The arms have on their early plates a transverse ridge which 

 is strong near the lateral margin and is faint or absent near the median 

 line. On successive plates this ridge becomes shorter and shorter, i. e., 

 apparently moves laterally on the arm like the nodes of C. reticulatus 

 until, between the thirtieth and fiftieth plates, it disappears altogether, 

 and the arm plates are smooth. These facts are here inteipreted to mean 

 that C. clarus is descended from C. prohoscidalis but diverges from other 

 lines of descent in the direction of loss of the ornamental feature repre- 

 sented l)y tlie transverse ridge. 



Having followed certain lines of descent from Cactocrinus prohosci- 

 dalis, it would be interesting to trace its ancestry, but I have, as yet, seen 



