LOWER AND UPPER GREENSAND OF THE SOUTH OF ENGLAND. 
445 
the margins are smooth (fig. 2), whilst in others the margins are sinuous and notched), 
(figs. 3, 3 a, 6). In the more complex forms the rays are divided and branched in an 
extraordinary variety of ways; in some cases the subdivisions are narrow (figs. 7, 7a), 
whilst in others they are mere lobate extensions (figs. 12, 126). The greatest modifi¬ 
cation occurs in the spicules, which are simple horizontal discs with lobate (fig. 136), 
jagged (13c), or smooth margins (14, 14a). In these all traces of the typical trifid 
spicule have disappeared in the external form, but the minute three-branched canal 
present in some of these discoid spicules (fig. 14) indicates very clearly that they have 
originated from the same fundamental type as the branched dermal spicules. As a 
general rule the canals in these dermal spicules extend only a short distance from 
their centres (figs. 10a, 11), and do not reach the extremities of the subdivided rays. 
In but slightly modified examples, however, the canals normally extend to the 
extremities of the rays (figs. 2, 3, 3a). 
These dermal spicules vary in diameter from '316 to ’816 mm., and the rays are 
from ’024. to T mm. in width. They are as a rule much larger than the average 
dermal spicules of recent lithistids, as may be seen from those of Discodermia sinuosa, 
Carter (figs. 15, 15a), which have been figured on the same scale as the fossil forms 
for comparison. 
These dermal spicules belong in all probability to many distinct species of lithistid 
sponges, but they are so rarely found in their natural position on the surface of these 
sponges in the fossil state, that very little is known at present of the particular forms 
belonging to each species. The only lithistid sponge from the English greensand in 
which the dermal spicules have been found in position, is Hallirhoa agariciformis, 
Benett, sp.,* and they are of the lobate character shown in figs. 12 , 126, 12 d. As 
numerous examples of the tetracladina genera Siphonia, Hallirhoa, Ragadinia, 
kalpmella, and Rhopalospongia are known from the greensand, which, there can 
be no doubt, possessed a definite spicular dermal layer, we may conclude that these 
dermal spicules belong to these genera. 
The dermal spicules are very generally distributed throughout the greensand, but 
they are more particularly abundant in the lower greensand at Haslemere, and in the 
upper greensand at Blackdown, Haldon, and Warminster. 
Order Haxactinellidce, O. Schmidt. 
In contrast with the abundance of the detached spicules of tetractinellid and lithistid 
sponges in the sponge-beds, those of the above order are few and of rare occurrence. 
There are fragments of the skeletal mesh of two species, in which the nodes are 
compact; in one (Plate 45, figs. 7, 7a, 6) the rays are robust and the distance from 
node to node is ‘3 mm.; in the other (fig. 8) the mesh is smaller and the nodes are 
* See Catalogue Fossil Sponges in the British Museum, pi. 15, fig. 15. 
MDCCCLXXXV. 3 a,] 
