ISCHADITES. 119 
anchoring-rods of H. Smithii, and in the absence of any other distinguishing cha- 
racters they may be included in this species. The larger spicules considerably 
exceed in size those in H. fasciculus, M‘Coy sp. The only examples of this species 
are in the cabinet of Professor C. Lapworth, F.G.S.; the microscopic section from 
them, which has been figured, is in the possession of the author. 
Distribution.—Ordovician. In a light grey limestone at Knockgeiran, near 
Girvan, Ayrshire. 
Family.—ReEcervacuitip”. 
Genus.—Iscuaptres, Murchison. 
1839. Silurian System, p. 697. 
Syn.—Tetragonis, Hichwald, Giimbel (in part), F. Roemer (in part), and other 
authors. Receptaculites, Hichwald (in part), Schmidt, Billings (in part), Meek, 
Worthen, Hall, Whitfield, and others. Selenoides, D. D. Owen. 
Sponges conical, ovate, subspherical, or pyriform, with conical, sometimes 
slightly elevated summits, in which there is a circular perforation opening into a 
central cavity. The distal or summit ray of the hexactinellid spicules forming the 
skeleton is modified into a delicate rhomboidal plate, which rests upon the four 
transverse or horizontal rays, whilst the ray extending inwards at right angles to 
che surface gradually tapers to an acute point. The spicules are disposed so that 
the summit plates are nearly in contact, and form regular spiral curves, extending 
from the basal nucleus to the summit of the Sponge, thus presenting an appearance 
like the engine-turned case of a watch (Plate II, fig. 2a). The transverse hori- 
zontal spicular rays overlap each other, but are not united together, and they 
divide the surface of the Sponge beneath the summit plates into oblong areas. 
(Plate II, figs. 1, 1 a). 
Ischadites is distinguished from Receptuculites, Defrance, and Acanthochonia, 
Hinde, by its conical or ovate form enclosing a central cavity, and from Sphero- 
spongia, Pengelly, by the rhomboidal form of its spicular plates. 
In common with the allied genera of this family, /schadites has till lately been 
placed with the Foraminifera, though by Billings and subsequently by Salter it was 
regarded as a Sponge. 
I have’ already given in some detail the arguments in favour of the spicular 
structure, and therefore Sponge-nature of this genus and its allies, which appear 
1 «Quart. Journ. Geol. Soe.,’ vol. xl, p. 827. 
