Th. Mortensen: On the alleged primitive Ophiuroid, Ophioierms elegaus Bell. 13 



species tigris to Ophiothela. Two characters alone distinguish it from this genus, 

 viz. the naked skin covering the disk and the dorsal side of the arm, and the 

 absence of the dorsal arm piates. It may be doubted, whethcr these two characters 

 are sufficient for a generic distinction ; for the present, however, i am inclincd to 

 think that they are so, and accordingly the name Ophioteresis may be retained. I shall 

 give here an emendated diagnosis of the genus, the diagnosis given by Bell being 

 quite inappropriate, as has ben established by these researches. 



Ophioteresis Bell. Disk and arms covered by a thick, naked skin, which obscures 

 the large radial shields and the ventral arm-plates. Dorsal arm-plates wanting, 

 except at the base of the arm and on the growing joints at the point of the arm. 

 Side arm-plates very prominent, carrying the more or less hook-shaped spines. Tube 

 feet issuing from the sides of the arm, without tentacle scales, except on the grow- 

 ing joints near the point of the arm. Mouth structure as usual in the Ophiothri- 

 chidæ : the tooth-papillæ arranged in 3 or 4 vertical rows. 



To this genus belong the two species elegans Bell and tigris Lym., which will 

 perhaps prove to be identical^). 



Upon the classification of the Ophiuroids the results of these researches have 

 mainly a negative bearing. The generally aocepted division of the simple-armed 

 Ophiurids into Zygophiuræ and Streptophiuræ has been done away with ; we have 

 again only a number of families, which can merely be arranged side by side, without 

 any definite order. In the present state of our knowledge it is impossible to see what 

 their real affinities are. Nobody can say which faniily represents the more primi- 

 tive type — not to speak of the faet that the families themselves are partly 

 in a rather chaotic condition (except the Ophiothrichidæ, which on account 

 of their characteristic mouth structure appear to be a very natural group). That 

 Ophioteresis is in no way more primitive than the other Ophiothrichids is evident. 

 The same is, in my opinion, the case with another Ophiurid, which has been claimed 

 to be a very primitive form, viz. Ophiotypa simplex Koehier. I siiall not, however, 

 enter on a detailed discussion of the perplexing Ophiuroid-classification, but only 

 Ståle that it will scarcely be possible to arrive at reliable results until the 

 anatomy and development of a good number of representative types has been 

 thoroughiy studied. 



1 may shortly thus .summ;iri/,(! thr results of thesr rcHcjinhos : 



Ophioteresis does not lack the ventral platos; they are pn .sent as in 

 other Ophiurids, only obscurcd by the thick skin. Accordingly thcrc 

 is no open ambulacral furrow. — Dorsal piates and tentacle scalet 



') Sce AddiUonal Note, pag. 16. 



