76 BRITISH FOSSIL SPONGES. 
shaped dise 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, OcractineiLip Spicu.es.—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). Ina single instance an 
abnormal spicule occurs in which only three of the horizontal rays are developed 
(Pl. I, tig. 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, Astreospongia, F. Roem. By Ferdinand Roemer’ the spicules 
were regarded as only normally possessing six rays, but Zittel’ pointed out the 
presence of a vertical axis in addition to the horizontal rays. By this latter author, 
however, Astra@ospongia 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. HreraorinreLiip Spicutes.—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, Vholiasterella, 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 Asteractinella, 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, 4); 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, f). 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 « Lethea Pal.,’ 1 Th., p. 314, 1880. 
* «Studien ueber foss. Spongien,’ i, p. 59, 1877. 
