62 THE PALEONTOLOGY OF MINNESOTA. 
{Ischadites 
“The structure consists of spicules of various dimensions, regularly arranged in 
vertical and oblique rows, and held in position by the interlocking of their summit- 
plates and horizontal rays. Head-plates of the spicules delicate structures with 
smooth, flattened outer surfaces, thickest in the central portion where they connect 
with the horizontal rays, and gradually diminishing towards the margins, which are 
very thin. They have a generally rhomboidal outline, but in some parts of the speci- 
men 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 invariably, present in the spicular-plates of the basal portion, 
which have their angles, or those directed away from the basal nucleus, either trun- 
cate or with a slight notch, from which one of the horizontal rays projects and 
extends nearly to the center of the plate immediately in front. The plates forming 
the basal nucleus are also more elongated than any others. 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 great- 
est dimensions (2 to 4 mm.); they then gradually diminish in size towards the 
summit, and the smallest plates surrounding the summit-aperture are scarcely dis- 
tinguishable without a lens, measuring from .25 to 4 mm. in width, or about one- 
tenth of the diameter of the zonal plates. 
“Head-plates arranged in regular spiral curves which, starting in opposite direc- 
tions from the basal nucleus and extending to the summit, give to the surface the 
exact appearance of the engine-turned case of a watch. Kach 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 immediately 
in front of it. Thus vertical lines extending from the base to 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 center of the base, there is a 
series of eight 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 connecting the lateral angles would trace a 
small circle, with the proximal angles of the plates for its center. 
“As a rule the margins of the plates appear to fit closely and evenly to each 
other, but in some eases 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 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 
