DR. G. J. HINDE ON RECEPTACULITID A, 811 
The specimens are now only in the condition of casts, that is, of 
impressions in the shale of the structures of the organisms, which 
have entirely disappeared; and it is evident that they have been 
greatly compressed, and that they were originally subspherical or 
ovate in form. The details of the form in these specimens can only 
partially be ascertained; the basal extremity appears to have been 
obtusely conical without either stem or roots; the body gradually 
increased in diameter for about half the distance between the base 
and summit, and then gradually contracted to the summit, at which 
there was a small circular aperture, which originally opened into the 
inner cavity. The surface of these bodies is covered with delicate 
oblique ridges, crossing each other with the greatest regularity, and 
dividing the surface into small rhomboidal depressed areas, disposed 
in quincunx. These areas are small near the base, widest in the 
central zone, and again minute towards the summit. Within the 
rhomboidal areas, and only occasionally shown, are two sets of lines, 
one set running straight from the base to the summit, and the other 
crossing the first at right angles, so that each rhomboidal area is 
divided into four parts by the lines which intersect in the centres of 
the areas and extend to each of their angles. These lines in 
the type specimen are only faintly and rarely seen, though in Mur- 
chison’s figures, which are partially restorations and not accurate 
representations of the originals, they are clearly delineated. 
The above is all that can be learned from an examination of 
Murchison’s type specimen of IJschadites Kenigii, which is now in 
the Museum of the Society. It seemed desirable to refer to the 
type form before passing to a description of the structural details of 
the genus, as shown in examples of the same species from Silurian 
strata of the isle of Gotland, in which the component parts of the 
organism are beautifully preserved in calcite or iron peroxide. 
Outer form.—This is to a certain extent variable even in the same 
species from the same locality. The prevalent forms are ovate or 
biconvex, or more or less depressed conical; some are subspherical, and 
others pyriform. The central zone in some specimens is bulged out 
conspicuously (Pl. XXXVI. fig. 1), but generally there is an evenly 
rounded contour from the base to the summit. The base may be 
either obtusely conical, flattened, or very frequently concave. The 
summit is usually obtusely conical; rarely there is a small central 
elevation (Pl. XX XVI. fig.1a). A small circular perforation is present 
in the centre of the summit which opens into the originally hollow 
cavity of the body; this is now invariably filled with the matrix of 
shale or mudstone. 
Structure.—This entirely 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. We 
will first consider the plates which form the summits or heads of the 
spicules. These are apparently delicate structures with smooth 
flattened upper or outer surfaces, thickest in the central portion 
where they connect with the horizontal rays, and gradually dimi- 
nishing towards the margins, which are very thin. They have a 
