from the Devonian of Cormcall. 163 



The disc is best preserved in the interradius v-i. It certainly 

 •extended up the rays to the distal end of the first segment (d in 

 Fig. 2), and perhaps a little further, but whether its outline was 

 straight or concave interradially cannot be decided. It was clothed 

 with minute spines radiating from the centre. 



The following are some measurements of the specimen in 

 millimetres : — 



Greatest length of the portion preserved 11*5 



Diameter of disc ... ... ... ... ... ... circa 6*5 



From centre to distal end of mouth -skeleton circa 2*0 



Width of ray 1-5 



Length of lateral spines, at least 1-8 



Average lengtlLof a segment ... ... '76 



Interpretation of the Holotypb. 



Owing to the difficulties of interpretation, the foregoing description 

 of the rays has been drawn up in as objective a fashion as possible. 

 Some interpretation, however, is a necessary preliminary to any 

 discussion of the relationships of the fossil. 



The median body of each segment is undoubtedly the equivalent 

 of the normal Ophiurid vertebra. On the accepted theory that this 

 vertebra was evolved by the fusion of a pair of ossicles originally 

 alternating, then opposite, and finally joined along the middle or 

 radial line, the present specimen may be 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. 

 Those 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. (See Note at end.) 



It was the existence of the median groove that first suggested the 

 determination of the exposed surface of the fossil as ventral. But 

 since such a groove may occur also on the dorsal surface of a vertebra, 

 that evidence is inconclusive. The determination is confirmed, how- 

 ever, by the conspicuousness of the subcircular excavations, or podial 

 pores, in the side-processes, and by the apparent continuation of 

 this groove over the radial angles of the mouth-frames. 



As a rule among Ophiurids in this stage of evolution, each 

 vertebra 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 vertebrse. 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 proximal side-process ? To this question two 

 possible answers present themselves. 



