I he Oe a eee re es 
from the Island of Malta. 83 
disc*: the posterior poriferous zone extends two-thirds of the 
distance between the disc and the border; the anterior zone 
commences much farther from the dise above, but extends as far 
as the posterior zone below; thus in the specimen before us, 
there are fourteen pairs of pores in the posterior, and only nine 
in the anterior zone: the postero-lateral ambulacral areas ex- 
tend backwards, and towards their termination they curve grace- 
fully outwards ; the angle they form at the transverse line at the 
disc is 62°, and there are twenty pairs of pores in each of the 
zones. ach of the interambulacral areas presents peculiar 
characters ; the antero-lateral pair are the smallest, and the pos- 
tero-lateral are the largest; they are both remarkable for the 
tubercles they support on their dorsal surface ; in the anterior pair 
there are nine, and in the posterior pair there are fourteen of these 
tubercles on each side of the test, which are sculptured out of 
the substance of the plates; as they do not project beyond the 
general surface of the shell, each tubercle is seated on a cylinder, 
which is surrounded by a deep, wide depression : as these perfo- 
rations are not arranged with much regularity, the test has the 
appearance of having been bored by some marine mollusk. 
The tubercles themselves, in proportion to the size of the sup- 
porting cylinder and encircling entrenchment, are small and per- 
forated; the single interambulacrum is narrow and elongated, 
and rises in the mesial line into a prominent elevated ridge; the 
posterior border is obliquely truncated downwards and inwards, 
in the upper part of this space the large anal opening is situated ; 
below, the interambulacrum forms another prominence marked by 
two concave depressions on each side thereof, from which a wide, 
gently sloping central space occupies the middle of the ventral sur- 
face, having the large transversely-oblong mouth-opening with its 
projecting posterior lip occupying the anterior third of the base ; 
on the sides of this sloping central space the basal portions of 
the postero-lateral interambulacra are thickly studded with 
large slanting perforated tubercles, arranged with much regu- 
larity in this region, and a few smaller tubercles are scattered 
over the basal portion of the antero-lateral pair; the crescentic 
depressions below the anus have each a group of perforated 
tubercles, and between them and the smooth central concave 
portion of the base is the subanal fasciole, which forms a trans- 
verse band in the middle, and a crescent on each side, the upper 
* It may be as well to state, that we have adopted this mode of mea- 
suring the amount of divergence of the antero-lateral and postero-lateral 
areas rather than the vague expressions formerly in use. The angle is 
measured from a transverse line which cuts the longitudinal axis at right 
angles at the apical dise; we have found the degree of divergence to be 
very uniform in each species. 
