1892.] CLASSIFICATION OF OPHITJROIDS. 179 



the saddle-shaped face of the Astrophytid ossicle is not seen here ; 

 we have merely a generalized Ophiurid ossicle, without knobs or pits. 



The most remarkable character of this Ophiurid (see Plate XI. 

 figs. 3 and 4) is the complete absence of a ventral plate ; no other 

 existing brittle-star is known to want this plate. The upper plates 

 are definitely double, and the side-plates, instead of lying flat against 

 the side of the central ossicles, are wider than long and stand out 

 from the sides of the arm. The radial shields are very large and 

 extend almost to the centre of the disk ; they have the form of right- 

 angled triangles, the hypothenuses of which face, but do not touch, 

 one another ; there are no other plates on the surface of the disk. 



It is necessary to form a new genns for this form, which may be 

 called Ophioteresis '. 



Definition of the Genus and Species. 



Ophioteresis is a streptospondyline Ophiurid in which the cover- 

 ing-plates of the arms are double above, wanting below, and wedge- 

 shaped at the sides ; the radial shields are well developed, and there 

 are ordinary teeth and teeth-papillae. 



Ophioteresis elegans has the disk more or less distinctly pentagonal, 

 of moderate size ; arm-spines five. Elegantly coloured, the upper 

 surface of the arms and the margins of the disk green, the central 

 portion of the disk dark, with an irregular pattern of meandering 

 white lines ; interradial portions of lower surface of disk dark, with 

 white lines ; the rest of the lower surface yellow. 



Hab. Seychelles, 4-12 fms. In coll. B. M. 



From this simple form differentiation would seem to have preceded 

 along two lines ; there has been an increase in complexity of articu- 

 lation, associated with the fixation of certain ossicles and spines, or 

 there has been vegetative repetition and branching with a more 

 primitive inconstancy and irregularity of anatomical characters. 



Around the primitive stock some forms — those which Mr. Lyman 

 calls the " Astrophyton-like Ophiurans " — have remained, such as 

 Ophioscolex, Ophiobyrsa, Neoplax, and Ophioteresis. 



4. The Subdivisions of Ophiuroids. 



It will perhaps be found convenient to give distinctive names to 

 the three groups; for brevity's sake I add here the definition of 

 Ophiuroids which I ventured to publish last September ^ 



The Ophiuroidea are cahculate, actinogonidial, eleutherozoic, 

 azygopodous Echinoderms, in which there is no distinct ambulacral 

 groove. The " arms " are sharply marked off from the disk, are 

 very rarely more than five in number, and are sometimes elaborately 

 branched. The digestive system, which is aproctous, and the gene- 

 rative are confined to the area of the disk, as is also the speciahzed 

 respiratory apparatus, which takes the form of deep clefts. 



The S'treptophiurse are Ophiurids in which the ambulacra] 



1 Tijpriais, alertness. '^ T. e, p. 214. 



