22 0. F. JEXKIN. 



infrequently that spicules may be found with one of the paired rays pointing almost 

 radially inwards, and at first sight suggesting that sub-dermal triradiates are present 

 (cf. Fig. 59). The peculiar shape of the dermal triradiates, however, makes it easy 

 to prove that it is always a paired ray, and not the basal ray, which is centripetally 

 directed. 



DERMATRETON CHARTACEUM. 

 (Plate XXVIL, Fig. 5, and Plates XXXI. and XXXII., Figs. 59-64.) 



There is only one specimen of this new species in the collection. It was found at 

 a depth of 180 fathoms. It is nearly straight, 23 mm. long x 3 mm. maximum 

 diameter, quite white as preserved in alcohol, and has the appearance of a delicate paper 

 tube (see Fig. 5). The walls are only 32 mm. thick at their thickest point. The 

 reticulated pattern on the surface is too small to be^readily seen with the naked eye. 



The structure of the body wall is shown in Fig. 59. The flagellated 

 chambers open directly into the gastral cavity through contractile apopylcs (see 

 Fig. 60). The skeleton of the flagellated chambers consists normally of a single 

 row of sub-gastral triradiates, but there is occasionally a smaller triradiate half-way 

 between the gastral and dermal layers. There are also fine hair oxea, not shown in 

 the figure, surrounding the flagellated chambers. The distal ends of the flagellated 

 chamber skeletons are strengthened by oxea, which project a short distance beyond the 

 dermis. The distal ends of the chambers are covered by a reticulated cortex with a 

 skeleton formed of curved triradiates (see Fig. 63). 



The gastral skeleton (Fig. 62) is formed of large quadriradiates, among which 

 there are a very few which appear to have no apical ray. 



The oscular skeleton is shown in Fig. 61. The edge, which is slightly 

 everted, is formed of small quadriradiates. There is a very short fringe of hair oxea, 

 amongst which are a few stout oxea similar to those in the body wall. The dermal 

 triradiates extend up to the edge. 



Spicules (Fig. 64). 



Oxea. There are two sorts of oxea : 



(a) Projecting oxea, slightly crooked, slightly hastate, 130/u to 270 n long 



x 11/u to 16/x thick. 

 (ft) Hair oxea about 1 /j. thick. 



Triradiates. The triradiates are of two sorts : 



(c) Alate sub-gastral triradiates forming the tubar skeleton. Basal ray straight, 

 tapering to a moderately sharp point, 320 p. to 700 / long x 9/* to 10/u 

 thick. Paired rays nearly equal, slightly crooked, 100,u to 160/ long 

 X 9 M to 12 M thick, bluntly pointed. Oral angle variable from 170 to 

 120 ; strongly folded ; angle of fold 120 to 130. 



