248 The American Naturalist. . [Mareh, 
small number of brachials below the free arms is also very notice- 
able when compared with the four groups yet to be considered. 
Teleiocrinus departs from the type just mentioned in having a 
greater number of the lower brachials incorporated into the calyx, 
and forming a more or less pronounced decagonal rim just above 
those of the second order. In this respect it approaches some- 
what towards Strotocrinus, but the latter has a very different 
ventral structure. Physetocrinus and Strotocrinus both differ 
from Actinocrinus in the structure of the ventral side, while the 
anal opening is a simple aperture in the test. The first of these 
types has the ventral portions of the calyx greatly elevated; the 
second nearly flat, while the rim is enormously developed, and 
the terminal free arms are not given off until the twelfth to 
fifteenth order of brachials. The calyx of Steganocrinus is most 
like that of the lobed section of Actinocrinus, but the radial ex- 
tensions are most remarkable, and give rise to a very large num- 
ber of free arms. 
Geologic Development.—Inasmuch as the different phases passed 
through during the known existence of several of the genera 
mentioned have already been referred to elsewhere,’ it is hardly 
necessary to take up here each group sepapately. It will suffice 
simply to consider somewhat in detail the geologic history of one 
of the leading generic types——Actinocrinus,—which will also 
indicate the general course of development pursued by the other 
members of the family. 
As yet the genus Actinocrinus is not known before the earliest 
part of the Lower Carboniferous—the Kinderhook. The forms 
from this horizon thus far discovered have all a more or less 
globular calyx, with the arms equidistantly distributed. The or- 
namentation has already assumed two very distinct phases. In 
the one, delicate ridges or small confluent nodes pass from the 
central portion of each dorsal plate of the calyx to the center of 
adjoining ossicles ; in the other, the ridges are very inconspicuous, 
and the plates are strongly convex on the outer surface. These 
two styles of sculpturing continue during the existence of the 
group; but the first gradually loses its identity, while the second 
2 Wachsmuth and Springer, Proc. Acad. Nat. Sci., Phila., 1878. 

