DR. G. J. HINDE ON RECEPTACULITIDS. 813 
intermediate space, now filled with the matrix. That the plates, 
or at least those of the lower portion of the organism, did not fit 
so closely as to exclude the passage of water from the exterior to 
the interior cavity of the organism, is shown by the fact that one of 
the horizontal spicular rays projects from underneath the distal 
angle of each of the plates and extends over the outer surface of 
the plate in front, thus clearly preventing a close-fitting union at 
the margins, and, further, the ridges, which characterize the outer 
surface of the casts of specimens, are produced by the infilling of 
the matrix in the interspaces between the margins of the plates. 
Though the spicular plates in some of the Gotland examples appear 
as if cemented together at their margins, yet a very slight degree 
of weathering is sufficient to show that there was originally a di- 
stinct, though minute, linear interval between them; and their 
apparent union is probably caused by the calcitic replacement of the 
originally separate structures which gives the appearance of their 
having been fused together into one mass. 
These summit- or head-plates appear to have been connected by 
a somewhat narrow neck to the horizontal rays of the spicules at 
the central point of junction with these and the vertical rays, as the 
horizontal rays appear to be independent except at their central 
junction. Asa rule, the head-plates are seldom preserved in situ. 
Thus in the large majority of the specimens from Gotland, they 
have quite disappeared, but in a few from a single locality, Djupvik, 
they are still retained. That they were present in all originally is 
made clear by faint traces of their marginal outlines which can 
generally be detected, and their absence is to be attributed to the 
facilities which their extended surfaces offer to weathering in- 
fluences. 
It has been already mentioned that the surface of the fossil 
immediately beneath the rhomboidal spicular plates is divided into 
minute oblong rectangular areas by vertical and concentric lines. 
These lines are formed by the apposition of the horizontal spi- 
cular arms or rays. ‘The spicules, in addition to the head-plate, 
consist of five rays; four extended in a horizontal direction, at 
right angles to each other, whilst the fifth extends from the junction 
of the four with the summit-plate towards the interior of the 
organism and thus at right angles to the horizontal rays. The 
spicular rays are circular in transverse section, thickest at their 
central point of junction with each other and the head-plate, and 
they gradually taper to bluntly-pointed extremities. Only in one 
specimen have I been able to detect the presence of canals in the 
interior of the rays. The vertical or entering ray appears to be the 
longest, the lateral rays are subequal, whilst the distal ray, or that 
pointing to the summit of the specimen, seems to be longer than the 
opposite or proximal ray. 
The four horizontal rays are so arranged that each ray extends 
towards one of the angles of the head-plate of the spicule. Thus one 
ray, the-proximal, points to the basal nucleus, and its opposite, the 
distal, to the summit. ‘This distal ray in the basal portion of the 
