ERYLUS OX VASTER. 



271 



600 /I long, and about as thick as the rhabdome of the regular dichotriaenes. 

 In such rhabdonies the axial thread terminates some distance from the aclad- 

 omal end, in the centre of its hemispherical surface. In the dichotriaene- 

 derivates with reduced clade number, the central parts of the axial threads of 

 both the rhabdome and the clades usually exhibit considerable irregularities. 



The oxyasters (Plate 3, figs. 32e, d, 33d, 34c, 35b, d; Plate 4, figs. 26d, 27b, 

 28c, 29f, 30d, f, 32-34e, 38-40) usually have a slight central thickening. This 

 is most clearly discernible in the monactine (Plate 4, fig. 27b) and diactine forms 

 (Plate 3, fig. 35b; Plate 4, fig. 28c). The rays are from one to twenty or more 

 in number, concentric, regularly distributed, and usually equal in size. They 

 are perfectly smooth (Plate 4, figs. 38-40), conical, and pointed. Very rarely 

 one or two rays are reduced in length, much shorter than the others, and termi- 

 nally rounded (Plate 3, fig. 34c). The properly developed rays of the small 

 oxyasters (Plate 4, figs. 39-40) are uniformly attenuated towards the pointed 

 end, those of the large ones (Plate 4, fig. 38) attenuated more rapidly in their 

 distal than in their proximal part. The rays are 6-55 jli long and, at the base, 

 0.8-4.5 /« thick. The whole aster measures 10-90 /x in diameter. The ray- 

 number is, as the following table shows, in inverse proportion to the size of the 

 rays and, apart of course from the monactines, of the whole aster. 



As mentioned above there is a kind of gap in the series of these oxyasters, 

 produced by the scarcity of forms 20-35 fi in diameter with from nine to eleven 

 rays. 



Most of the aspidasters (Plate 4, figs. 35-37, 41-43) are stout oval discs. 

 Very rarely roundish (Plate 4, fig. 41) or irregular aspidasters have been observed. 

 The ordinary oval aspidasters are 208-243 p. long, 125-150 p broad and in the 

 middle 30-40 /x thick. Towards the margin they thin out gradually. The 

 average proportion of length to breadth to (central) thickness is 100 : 63.3 : 15.7. 



