1881.] NATURAL SCIENCES OF PHILADELPHIA. 217 



the Slrotocrinus rim is alreadj' indicated, and in specimens in 

 which the arms are preserved, their lower portions stand out hori- 

 zontall}^ as in that genus; but the plates of these parts are not 

 connected laterall}^ which feature, aside from the difference in the 

 number of arms, constitutes the principal distinction between 

 Actinoci'inus and Teleiocrinus. 



Throughout tliis group, all bifurcations of the ray — after the 

 first — take place on the first plate in each order, only one of the 

 branches dividing again, and this alternately from opposite sides, 

 the other branch remaining simple. The arrangement is such 

 that the bifurcating plates of each primary division of the ray 

 follow each other in direct succession, forming two main trunks, 

 while the plates which remain simple, and are succeeded by others 

 to the edge of the rim, are given off alternately like pinnules. 

 These lateral branches are separated from the main rays by small 

 pieces, and each branch supports a free arm at the edge of the 

 rim. Within the rim, the radial series are conspicuousl}^ marked 

 by sharp carinae or ridges, which pass from plate to plate, and 

 follow both main and lateral divisions, while the small accessor^^ 

 pieces, which connect them, are formed into deep depressions. 

 Comparing the ridges with the elevations we have described in 

 Glyptocrinus^ and which are found in other Silurian genera, the 

 resemblance is indeed very striking. The ridges in the latter 

 extend over the primary-, secondar}', and sometimes over the ter- 

 tiary radials, and pass gradually into arms ; but while we find in 

 Glyptocrinus very strong arm-like pinnules, there are in Slroto- 

 crinus and Teleiocrinus pinnule-like arms, both included within 

 the body walls, and both springing off laterally like ordinarj'^ pin- 

 nules. The lateral branches in the rim of Slrotocrinus were 

 evidently pinnules in the young animal, and free as in the younger 

 stage of Glyptocrinus^ but with growth gradually developed into 

 regular arms; while those of the latter remained as pinnules 

 during lifetime. This explanation accords with the construction 

 of pinnules, which is so similar to that of arms, that it is in 

 many cases exceedingly^ dilficult to draw a line between them. In 

 Melocrinus the alternate pinnule-bearing appendages were called 

 by some authors arms, by others pinnules. The branches in 

 Cyathocrinus were called pinnules by Wyville Thomson, and arms 

 by most other authors. 



Following out the observations, it seems probable that all arms 



