218 PROCEEDINGS OF THE ACADEMY OF [1881. 



above the first bifurcation are metamorphosed pinnules, given oft' 

 from the primary radials. Applying this rule to the case of 

 Actinocrinus and Teleiocrinus, the idea is suggested that A. pro- 

 boscidialis, which is the most common species of the group, and 

 has only four arms to the ray, is in all probability the progenitor 

 of all similar Burlington species, evidently of both genera. In 

 A. recticulalus only the proximal pinnule toward the posterior 

 side was transformed ; in A. clarus one pinnule in each ray ; in 

 species with six arms the first pinnule on the opposite side was 

 added ; in species with seven arms the first and second pinnules 

 of one side, and the first of the other, and so on alternately on 

 opposite sides. Although the increase of arms is frequently 

 attended with some irregularity, the number of arms should here 

 be considered of specific importance, and deviations from the 

 normal number as intermediate steps between the species. 



During the Lower Burlington Limestone epoch, the number of 

 arms never exceeded eight to the ray, but in species even of that 

 number the arms are so crowded together, that they could not 

 have been arranged side by side, were not their lower portions 

 bent outward, in the same direction as we find the rim in Stroto- 

 crinus. In s[)ecies of Actinocrinus with only a few arms, the 

 arms are movable from the base up ; movement is less free in 

 species with six arms to the ra}^, and the facilit}^ of motion is 

 lessened with every increase of arms. This lack of mobility, of 

 course, onl}^ extended to the lower arm joints, which for some dis- 

 tance were so closely crowded together that they could not have 

 moved in any direction, and it was probably in consequence of this 

 inactivity that the proximal arm pieces, which in the simpler forms 

 were free, became gradually connected by growth. This was 

 evidently the first step in the direction of Teleiocrinus. After- 

 wards, by still further increase of arms, additional plates became 

 laterally attached, and in this way the rim was gradually developed. 

 In Actinocrinus the rim was mereh^ indicated by the adhesions of 

 a few plates to the calyx, the primary rays, and their main divisions 

 being still distinctly separate. In Actinocrinus (Strofocrimis) 

 serratus^ Meek and Worthen, which forms a kind of connecting 

 link between Actinocrinus and Teleiocrinus^ only the two main 



' This is the only species from the Lower Burlington beds which might 

 be referred to Teleiocrinus. 



