DR. G. J. HINDE ON RECEPTACULITID &. 815 
In a similar manner, Giimbel has figured a vertical section of an 
Ischadites * from Gotland, in which a dark shading is intended to 
indicate the outline of an inner layer, but, from the form and ex- 
tension of the spicular rays, the figure is evidently diagrammatical, 
and I have no doubt that, as in the Gotland specimens which I 
have examined, the spicular rays terminate freely. 
Not unfrequently examples of /schadites occur, in which, through 
weathering, both the spicular head-plates and the horizontal rays 
have disappeared, and only the basal ends of the vertical rays are 
shown on the surface as small rounded nodes, disposed in vertical 
lines (PI. XXXVI. fig.1¢). Ihave also noticed, in some specimens 
in which the walls have been crushed in, spicules completely detached 
and removed from their original positions, thus showing clearly their 
independence. 
From the characters of Jschadites as given above, there can be 
no doubt that the genus Tetragonis, Hichwaldt, is congeneric 
therewith. From the descriptions and figures of the type species, 
T. Murchisoni, Kichw., it is evident that it is a specimen in which 
the spicular head-plates have entirely disappeared, and the surface 
exhibits the small oblong areas formed by the horizontal rays, 
together with the apertures of the vertical rays at their angles. 
Eichwald himself recognized a relationship to Ischadites, and en- 
trusted his type specimen to Murchison for comparison with this 
latter genus; but in the ‘ Lethea Rossica’t, he makes no further 
mention of this, although he therein describes some forms of Jscha- 
dites, and also includes under Tetragonis two new species, 7’. sul- 
cata and T. parvipora, which, however, belong to an entirely 
different group than the type species. By most later writers the 
generic identity of Tetragonis and Ischadites has been freely ac- 
knowledged ; Ferd. Romer §, however, expresses doubts as to the 
former existence of plates and of the apertures at the angles in the 
type specimen, but this mistake has probably arisen from regarding . 
as examples of the genus, other forms which have no close relation 
to it. 
As the term Jschadites, Murch., has the priority of publication the 
name Tetragonis, Kichw., becomes obsolete. 
The genus Jschadites itself has, by several writers, been regarded 
as identical with Receptaculites, but though similar in its main 
structural features to this latter genus, it is sufficiently characterized 
by its conical or ovate form, inclosing a central cavity, with a small 
summit-aperture, and by the absence of an inner layer. From 
Spherospongia, Pengelly, it is distinguished by the rhomboidal form 
of the spicular plates, and the development of vertical spicular rays ; 
and from Acanthochonia, gen. nov., by its conical ovate form and 
central cavity. 
* Beitr. Taf, A. f. 29. . 
+ Urwelt Russlands, Heft 2 (1842), p. 81, t. 3. f. 18. 
t Vol. i. pt. 1, p. 480. § Lethza Pal. Th. i. p. 303. 
