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. — Receptaculitid^b. 



Genus. — Ischadites, Murchison. 



1839. Silurian System, p. 697. 



Syn. — Tetragonis, Eichwald, Gilmbel (in part), F. Roemer (in part), and other 

 authors. Receptaculites, Eichwald (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. 2 a). 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 Receptaculites, Defrance, and Acanthochonia, 

 Hinde, by its conical or ovate form enclosing a central cavity, and from Sphairo- 

 spongia, Pengelly, by the rhomboidal form of its spicular plates. 



In common with the allied genera of this family, Ischadites has till lately been 

 placed with the Foraminifera, though by Billings and subsequently by Salter it was 

 regarded as a Sponge. 



I have 1 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. Soc.,' vol. xl, p. 827. 



