1896. ] OF THE PALHOZOIC OPHIUROIDEA, 1029 
retained Joh. Miiller’s two orders, as he did also in his latest 
paper. 
In 1892 a short and pregnant paper by Prof. F. J. Bell [1] 
lifted the classification of the Ophiurids on to a different plane. 
Bell recognized the great importance of the vertebral ossicles and 
that they are of three main types: (1) the “ streptospondyline,” 
where the vertebral ossicles articulate by saddle-shaped surfaces, 
which do not bear lateral processes or pits; (2) the “ zygospondy- 
line,” where lateral processes and pits on the articular surfaces of 
vertebral ossicles limit the power of movement; (3) the “clad- 
ophiuroid” (or astrophiuroid), where the vertebral ossicles articulate 
by hourglass-shaped surfaces. 
Bell therefore proposed to divide the Ophiurids into three 
groups: (1) the Streptophiure, for those with streptospondyline 
ossicles; (2) the Cladophiure, for those with hourglass-shaped 
“at oatret (3) the Zygophiure, for those with zygospondyline 
ossicles. 
The definition of these three orders was no doubt a great 
improvement on any previous arrangement of the Ophiurids. There 
is, however, considerable difficulty in applying this system to 
the fossil forms, especially in the case of the Streptophiure. It - 
appears doubtful whether even some recent genera, as Ophiohelus, 
can be correctly described as having vertebral ossicles articulating 
by ball-and-socket joints. But this statement certainly cannot be 
made of many Paleozoic Ophiurids, which represent a more 
primitive condition than that of the recent species; they are indeed 
so primitive that they cannot be made to enter into any of Bell’s 
orders. 
The two most striking characters of these Paleozoic genera are 
the absence of ventral arm-plates* and of true vertebral ossicles. 
The latter are represented by free paired plates, like the ambu- 
lacral ossicles of Asterids. 
The ambulacral ossicles are the most important plates in the 
arms of both Asterids and Ophiurids, so that it is @ priort probable 
that they offer a better basis for classification than the external 
arm-plates. As we descend from the Zygophiure, first to the 
Cladophiure, and then to the Streptophiure, we notice a decrease 
in the complexity and completeness of the vertebral ossicles. It 
is not therefore surprising, when we go back to Paleozoic times, 
to find Ophiurids with an arm-structure still simpler than anything 
found in the Streptophiure. In these early forms the central 
arm-ossicles occur as a double series of free plates, below which is 
an open ambulacral groove. Hence the arms appear, at first sight, 
to be Asterid rather than Ophiurid in arrangement. 
Hence I propose to found a fourth order of Ophiuroidea to 
include those without vertebral ossicles, but which have in each 
arm a double series of free ambulacral plates, which articulate like 
1 This character is also found in the genus Ophioteresis of Bell, one of the 
most primitive of living Ophiurids; it has, however, vertebral ossicles with 
streptospondyline articulations. 
