812 DR. G. J. HINDE ON RECEPTACULITIDA, 
generally rhomboidal outline, but in some parts of the specimen two 
of the sides of the rhomboids are not uniformly straight, but have a 
slight curve, which gives the plates the form of a sector of a circle. 
Another modification is frequently, if not variably, present in the 
spicular plates of the basal portion, which have their distal angles, 
or those directed away from the basal nucleus, either truncate or 
with a slight notch, from which one of the horizontal rays projects 
and extends, as will be shown, nearly to the centre of the plate 
immediately in front (Pl. XXXVI. fig. 1g). The plates forming the 
basal nucleus are also more elongated than any others (Pl. XXXVI. 
fig. 1f). At the intercalation of fresh rows of spicules, there is a 
modified summit-plate of a pentagonal form and larger size ; in front 
of this is a triangular plate, which is succeeded by plates of normal 
shape. The plates near the nucleus, as well as those of the nucleus 
itself, are relatively small, but they quickly increase in size towards 
the zonal area, where they attain their greatest dimensions (2 to 4 
millim.); they then gradually diminish in size towards the summit, 
and the smallest plates surrounding the summit-aperture are 
scarcely distinguishable without a lens, measuring from 25 to -4 
millim. in width, or about one-tenth the diameter of the zonal 
plates. In none of the examples of this genus have I detected any 
minute structure in the head-plates of the spicules; they are now 
throughout of crystalline calcite, and in some cases sufficiently 
thin and translucent to allow the horizontal rays to be seen through 
them. 
The manner in which the spicular plates are arranged on the 
surface of the organism forms its most conspicuous feature. They 
are disposed in regular spiral curves which, starting in opposite 
directions from the basal nucleus and extending to the summit, give 
to the surface the exact appearance of the engine-turned case of a 
watch. This arrangement has been compared to that of the plates 
of a ganoid fish, and the scales of the cone of a pine tree. Each 
rhomboidal plate is so arranged that one of its angles points to the 
basal nucleus, and its opposite angle to the summit of the specimen, 
whilst the other angles are lateral, so that the distal angle of one 
plate is in contact with the proximal angle of the plate imme- 
diately in front of it. Thus vertical lines extending from the 
base to the summit would pass through the proximal and distal 
angles of the plates, whilst concentric lines would pass through the 
lateral angles. At the nucleus, or centre of the base, there is a 
series of 8 minute spicules with diamond-shaped head-plates, which 
are so arranged as to form a star-shaped figure, the distal angles of 
each plate representing one of the rays of the star, and a line con- 
necting the lateral angles would trace a small circle, with the 
proximal angles of the plates for its centre (Pl. XXXVI. fig. 1 f). 
As a rule, the margins of the plates appear to fit closely and 
evenly to each other, so as not to leave any interspace between 
their edges, but in some cases the upper or front margins seem to 
be slightly elevated as if they imbricated over the lower or hind 
margins of the spicular plates immediately in front, and left a small 
