SPONGES FROM PORTUGAL. 215 



asters, few in number, with small centres and long rays, 

 0'016 mm. in diameter, occurring here and there in the 

 lowermost part of the specimen. (4) " Selenasters," 

 forming a thick layer below the spherasters with large 

 centres, in fact constituting the chief mass of the specimen. 

 These spicules correspond in structure and position to 

 the sterrasters of the genera Pachymatisma, Cydonium, 

 etc., and I have chosen the name from their faint resem- 

 blance to a half -moon. They are really more sausage-like 

 in shape and the generic name Physcaphora has been 

 adopted to express this. The full-grown spicule measures 

 0'08 by 0'028 mm., and fortunately a number of young 

 stages were met with, so that the development of the 

 spicule could be traced. In the youngest condition 

 present the spicules had the shape of rods, nearly straight 

 or slightly twisted, beset with minute spines (PL XIII., 

 fig. 2a). In the next stage the spicule is still pretty 

 straight, but the spines are large and numerous, although 

 still distinctly separated (PI. XIII. , fig. 2b). In the next 

 stage, the spicule has already its typical sausage-shape, 

 the spines are very closely set, but still recognisable in 

 their individuality (PL XIII. , fig. 2c). The last stage is 

 the full-grown selenaster, in which the spines, except 

 their most distal ends, are all fused so as to form one 

 solid mass (PL XIII., fig. 2d). The distal ends of the spines 

 project a short distance beyond the surface of the spicule, 

 and being polygonal, chiefly hexagonal in transverse 

 section, offer a delicate pattern, when the spicule is being 

 focussed at different depths. A hilus is present as in the 

 Sterrasters. We thus see a great resemblance in the 

 structure and development of Sterrasters and Selenasters. 

 The chief difference is that in the Sterrasters all rays 

 start from a point, whilst in the Selenasters the rays 

 start from a line. 



