REVISION OF PALEOZOIC STELLEROIDEA. 257 



radial lino, the present specimen may bo described as in a stage of 

 development later than the fusion, but possibly retaining in the 

 median groove a trace of the primitive distinctness of the ossicles. 

 Tliose ossicles are usually described as ambulacral, and that term 

 may be accepted so far as it denotes homology with the ossicles 

 so named in an Asterid. * * * 



''As a rule among Ophiurids in tliis stage of evolution, each ver- 

 tebra has on each side a single side process on which abuts an outer 

 ossicle, the so-called adambulacral, and these adambulacrals, which 

 bear the lateral spines, alternate with the ambulacral ossicles or 

 vertebrae. It is a little difficult to interpret the appearances of the 

 present specimen in harmony with such an arrangement. On the 

 assumption that each segment should contain one ambulacral pair 

 (here fused) and one adambulacral pair bearing spines, ray i has been 

 assigned nine segments. But each such segment is seen to have two 

 processes on each side, and the distal one of these bears the spines. 

 For this reason, and because of the appearances in segment 8, it may 

 be supposed that the outer spiniferous portion of the distal process 

 (c in fig. 1) is an adambulacral element. What, then, is the proxi- 

 mal side process? To tliis question two possible answers present 

 themselves. 



"The appearance of a transverse division in the vertebra, between 

 the proximal and distal processes, suggests that each segment really 

 consists of two ambulacral pairs, of which only the distal one is 

 associated with adambulacrals, the proximal pah" of adambulacrals 

 being suppressed. Each of these supposed ambulacral pairs pos- 

 sesses a single pair of lateral processes, but those of the distal pair 

 are partly overlaid by the adambidacral elements. In the distal 

 region of the ray the proximal and distal halves of the vert-ebra are 

 distmguishable; but nearer the disk and within it they have come 

 closer together, the transverse division has disappeared, and the 

 side processes are reduced to structures which appear as bars, but 

 are probably flattened laminas. Thus the vertebrae seen in ray v are, 

 on this explanation, of compound origin. * * * 



''Now, comparison of the vertebrte of recent Ophiurids with the arm 

 segments, especially the proximal ones, of our fossil leaves no room for 

 doubt that the depression marked p is for the reception of a podium, 

 while that marked q is for the insertion of the ventral intervertebral 

 muscles. A single arm segment of a recent Ophiurid, such as Ophi- 

 arachna or AmpTiiura, is therefore homologous with any one of the 

 whole segments here numbered 1-8. If one of these is really com- 

 posed of a distal and a proximal half (lettered a and h respectively 

 in figs. 1 and 2) the same statement would apply to the recent Ophi- 

 urid, and some trace of this origin might be seen in the development. 



