Th. Mortensen: On the alleged primitive Ophiuroid, Ophioteresis degans Bell. 11 



than a typical Zygophiuran like Ophwthrix. This result led me to examine 

 some other of the so-called Streptophiurids, whose vertebræ, according to Bell are 

 of a primitive t3rpe, articulating with one another by means of a more or less 

 simple ball-and-soeket joint, devoid of the lateral processes and pits, which limit 

 the movement of the vertebræ on one another. I have chosen two of the main 

 genera referred by Bell to the Streptophiuræ, viz. Ophioscolex and Ophiomyxa. 

 PI. II. figs. 13 — 17 represent diflferent views of the vertebræ of Ophioscolex glacialis 

 PI. II. Figs. 18 — 22 corresponding views of those of Ophiomyxa australis. A compa- 

 rison of these figures with the corresponding views of the vertebræ of typical 

 Zygophiurans like Ophiothrix fragilis (PI. II. Figs. 8 — 12) and Ophiomusium Lymani 

 (PI. II. Figs. 23 — 24) makes it quite evident that the alleged difference 

 between them with regard to the development of the articulation 

 does not exist at all. In all of them the articulation is performed by a 

 median knob with a pit at each side of it on the aboral surface of the vertebra, 

 a median pit and a knob at each side of it on the adoral surface ; this is the 

 main articulating structure, above which may occur more or less developed knobs 

 and pits; these latter appear to be very diversified and will perhaps be of value 

 for classification. The lateral processes have nothing to do with the articulation ; 

 they only serve for the attachment of the intervertebral muscles. The "articular 

 cavities" in the figure of Bell, reproduced in textfigure 1, accordingly have nothing 

 to do with the articulation ; they are the grooves for the attachment of the muscles, 

 while the articulating pits and knobs are not at all represented in that figure. 

 The figures given on PI. II give convincing proof that the subdivisions 

 ^* Streptophiuræ'''' and '*Zygophiuræ" lack real foundation, the differences upon 

 which they are established do not exist at all but are due to misunderstand ing 

 and superficial examination. The same pits, knobs and processes are found in 

 Ophioscolex and Ophiomyxa as well as in Ophiothrix, Ophiomusium and Ophioteresis^ 

 the former being in no way more primitive than the latter — on the contrary, 

 the articulating pits and knobs may perha|)s be said to be more developed than 

 in Ophiothrix. — The othor genera referred by Bell to the "Streptophiuræ" I have 

 not had the opportunity to examine, but as there has not been given the slightest 

 f)roof that their vertebral articulations are really more simple than those of the 

 other Ophiurids, I think we can safely depend on their agreement with tho other 

 Ophiurids in this respect. The figures of vertebræ of diffcrent forms of Ophiurids 

 given by Lyman in his ''Challenger^'-Monograph are upon the whole not very 

 good, but it appears that in all of them the structure is essen tially the same. 

 Another type of articulation is only found in the Astrophytids — but I shall not 

 enter hcre into more detail on this matter, contcnting myself with having given the 



