66 CEYLON PEAKL OYSTER REPORT. 



see very clearly how oxeote megascleres may be derived directly from primitive 

 triaxon or tetraxon spicules by suppression of one or two actines (see Plate II., 

 fig. 1). From the same source are derived microxea, and these, being frequently 

 curved, have probably given rise to the sigmata and toxa and, perhaps, other 

 curvilinear microscleres of higher groups. Increase in the number of actines of the 

 primitive spicule, on the other hand, accompanied by other modifications, has 

 probably given rise to the astrose series of microscleres. Thus Dercitopsis and its 

 allies may be regarded as representing at the present day the common ancestors of 

 both Astrophora and Sigmatophora. 



Dercitopsis ceylonica, n. sp. Plate II., fig. 1. 



This species is represented in the collection by two small, flat pieces which have 

 probably been encrusting and may have formed parts of the same specimen. One is 

 about 22 millims. in length, 13 millims. in greatest breadth, and 7 millims. in greatest 

 thickness. The other is about 26 millims. in length, 16 millims. in greatest breadth, 

 and 7 millims. in greatest thickness. The surface is smooth but rather uneven. 

 Colour in spirit, dark slate grey on the outside, dull yellow internally. Vents 

 minute, scattered singly, with a tendency towards marginal arrangement or groujnng 

 on prominent parts. Inhalant pores dispersed abundantly over the surface, not in 

 sieves. 



Skeleton very dense, for the most part quite confused and irregular, but with the 

 smallest microxea forming a thin dermal crust, in which they are commonly arranged 

 at right angles to the surface. 



Spicules. (1.) Calthrops and triods (Plate II., tig. 1, a-h); rays smooth, sharply and 

 gradually pointed, commonly about - 033 millim. long by - 005 millim. in maximum 

 diameter, but subject to considerable variation, and occasionally exhibiting monstrous 

 forms such as twins. 



(2.) Oxea (fig. 1, i-6) ; varying enormously in size, but in such a perfectly 

 graduated series between the smallest and the largest that they cannot be divided 

 into mega- and microscleres. The dermal microxea measure only about - 025 millim. 

 by - 002 millim., while the oxea in the deeper parts of the sponge frequently measure 

 as much as 0'42 millim. by 0"012 millim. Both large and small oxea are smooth, 

 gradually and finely pointed at each end, and frequently centrotylote. They often 

 exhibit a curvature, which may be sigmoid or toxoid, and thus strongly support the 

 conclusion arrived at by Sollas (loc. cit., p. 109) to the effect that the toxa of 

 Dercitus are probably microxea which have acquired a curvilinear growth. The 

 same author has also pointed out that the spinose microrhabds of Dercitus originate as 

 smooth centrotylote microxea, which he regards as diactinal asters. From a similar 

 origin we get, in the case of Dercitopsis ceylonica, comparatively large oxeote 

 megascleres, which seems to indicate that this common type of spicule may also have 

 been derived from an aster. It also seems probable that the well-known sigmata, sy 



