76 BRITISH FOSSIL SPONGES. 



shaped disc with several incurved rays. The derivation of this form from the 

 normal hexactinellids is not apparent ; it has been suggested by Schultze, however, 

 that the incurved rays are only secondary processes of the nature of spines. 



5. Octactinellid Spicules. — In typical spicules eight rays are present, six of 

 which radiate in a horizontal plane from a common, slightly expanded centre at 

 equal angles from each other, whilst the other two rays form a vertical axis (PI. IV, 

 figs. 8, a — c). This normal type is not, however, of such frequent occurrence as the 

 modification, in which the rays forming the vertical axis are either reduced to small 

 blunt knobs or altogether absent (PI. I, figs. 7, a, b, c). In a single instance an 

 abnormal spicule occurs in which only three of the horizontal rays are developed 

 (PI. I, fig. 7, d). The rays of these spicules are uniformly simple ; the horizontal 

 ones appear originally to have been equal in length, and they usually terminate 

 obtusely. In weathered examples, open furrows are exposed on the surface of the 

 rays, indicating the presence of canals. These spicules have been recognised in 

 only a single genus, Astrceospongia, F. Roem. By Ferdinand Roemer 1 the spicules 

 were regarded as only normally possessing six rays, but Zittel 2 pointed out the 

 presence of a vertical axis in addition to the horizontal rays. By this latter author } 

 however, Astrceospongia has been placed with the Lyssakine hexactinellids, but the 

 number and disposition of the spicular rays differ so markedly from those of 

 hexactinellid spicules that it is difficult to understand how they can have been 

 derived from the hexactinellid type, and it seems preferable to regard them as 

 belonging to a distinct sub-order. 



6. Heteractinellid Spicules. — The above name is proposed for skeletal-spicules 

 with a variable number of rays, ranging from six to thirty, extending from a 

 common centre at different angles. In one genus, Tholiasterella, the spicules possess 

 from six to nine rays, projecting nearly horizontally from a central disc, and a 

 single ray extending at right angles from the centre of the disc. The rays may be 

 equal or unequal in length, usually simple, tapering, and blunted, and with numerous 

 projecting warts on their upper surfaces (PI. VII, figs. 1, c — g, 2, a — d). In the 

 genus Aster actinella, one form of skeletal-spicule has from eight to twenty rays 

 radiating in different directions from a common centre. One of these rays is 

 usually longer and more prominent than the others, which are unequal in size (Fig. 

 7, a) ; in another form of spicule there are as many as thirty rays, the greater 

 number of which are disposed side by side and partially amalgamated, so as to 

 form a nearly horizontal disc ; on the under surface of this there are three or four 

 rays diverging at various angles (PL VIII, figs. 3,e,/). Owing to the large size 

 and thickness of the rays, it has not been practicable to determine the character 

 of the axial canals. These spicules appear to fundamentally differ from the pre- 



1 ' Lethsa Pal.,' 1 TL, p. 314, 1880. 



2 ' Studien ueber foss. Spongien,' i, p. 59, 1877. 



