216 PROCEEDINGS OF THE ACADEMY OF [1881. 



On examining a large collection of the different species, it will 

 be found that a separation according to the number of arms is 

 not so satisfactory as might be expected, for only a few specimens 

 will be found — except the four-armed ones — which agree with the 

 given arm formula of the species. The majority will be found to 

 have in one or more rays very irregularly, either a surplus or a 

 deficiency of arms, and the greater the number of arms a species 

 possesses, the oftener such irregularities occur. The difficulty of 

 identifjdng these species is further increased by the similarity — 

 we might almost call it identity — of general form and ornamenta- 

 tion, which prevails throughout the group. 



The gradual increase of arms would naturally lead us to inquire 

 whether it might be connected with the growth of these crinoids — 

 an idea which seemed at first plausible, inasmuch as the above 

 species are found exclusively in the Lower Burlington beds ; but 

 an examination of specimens, with the arms in place, shows that 

 such is not the case. Specimens with four arm openings in the 

 body to each ray, have also four simple arms, while they 

 should have, if representing a younger stage of the six- or eight- 

 armed species, the same number of arms as the adult, with the 

 bifurcations taking place beyond the body as in Platijcrinua. 

 The fact is, however, that the arms of A. proboscidialis are not 

 only simple, but from the base up, are composed of a double 

 series of pieces, while the plates which should form the higher 

 orders of radials in the adult are entirely absent. 



In Strotocrinus^ which is closely related to Actinocrinus, the 

 variations in the number of arms are still greater, being among the 

 different species from eight to twentj^-four to the ray. Strofocri- 

 nus has also been divided b}^ Meek and Worthen into two sections ; 

 the first including species with a simple anal opening directly 

 through the vault, which they call the typical form, and the second, 

 those with a large sub-central anal tube, for which we have pro- 

 posed the sub-genus Teleiocrinus. The ornamentation among the 

 different species of the two sections is remarkably similar, only 

 that in some species the striations are more prominent, in others 

 the nodes. The most important feature of the genus is the pecu- 

 liar rim, which extends out horizontally from the body, formed of 

 the higher orders of radials, which are connected by interradial, 

 interaxillar}^, and some other apparently accessory pieces. In the 

 allied Actinocrinus spinotentactdiis with eight arms to the ray, 



