SPONGES, GRAPTOLITES, CORALS. 63 
Ischadites.] 
to exclude the passage of water from the exterior to the interior cavity of the organ- 
ism, is shown by the fact that one of the horizontal spicular rays projects from under- 
neath the distant 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. 
“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. 
“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 spicular 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 trans- 
verse section, thickest at their central point of junction with each other and the 
head-plate, and they gradually taper to bluntly-pointed extremities [In Ischadites 
iowensis they are needle-shaped|. Canals present 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 organism frequently projects beyond the margin of the spicular 
head, and overlies the head-plate of the spicule immediately in front or above it. 
“The vertical rays of the spicules, which extend at right angles to the summit- 
plates and the horizontal rays, are only seen when the interior of the specimens is 
exposed by fracture or by section. They appear as delicate, gradually tapering 
shafts, the extremities of which are pointed, and reach about half way to the center 
of the interior cavity, where they terminate freely. An interior plate or integument 
corresponding to that in Receptaculites has not been observed. 
“The genus Ischadites itself has, by several writers, been regarded as identical 
