822 DR. G. J. HINDE ON RECEPTACULITIDA, 
Taking first into consideration the structure of the outer or under 
surface of the fossil, we find that it consists of a layer of rhomboidal 
spicular plates of the same character and disposed in precisely the 
same manner as those in Ischadites and Acanthochonia. I have not 
been able to ascertain definitely the precise form of the nucleus or 
commencement of growth of the spicules, but it is probably similar 
to that of the genera already described; the plates in the centre 
are the smallest, whilst those in the central and peripheral portions 
of the organism are the largest. The outer surface of the spicular 
plates is flat, and their margins are very thin and delicate; the 
central portion was firmly united to the rest of the spicule imme- 
diately above the horizontal rays. There appear in this genus also 
delicate linear interspaces between the margins of the plates, which 
are particularly conspicuous in silicified specimens which have been 
freed from the calcitic matrix; the interspaces, however, in these 
forms may be partly owing to the destruction by the acid of the 
delicate edges of the plates. No structure is visible in the summit- 
plates ; when silicified they are apparently compact, whilst in the 
calcareous forms, when the surface is gently rubbed and polished, 
radiating fibrous crystals of calcite, oftentimes intermingled with 
iron-peroxide, are exposed. 
According to Giimbel *, the spicular plates (Kalkplattchen) of 
Receptaculites consist of three distinct layers—(1) of a thin carbo- 
naceous layer, (2) of an upper layer of crystalline calcite, and (3) of 
an under layer of the same material; and there seems therefore a 
notable discrepancy between his observations and my own that the 
plates are composed of a single, relatively thin, structureless layer. 
This discrepancy is, however, more apparent than real, since the 
spicular plates as I have defined them correspond to only the second 
or upper calcareous layer of Giimbel. The carbonaceous surface- 
layer, which Giimbel describes as extending completely over both 
the outer and inner surfaces of the organism, appears from his 
description to be of a very dubious character. From what I have 
seen of the Ober-Kunzendorf specimens in which it is said to occur, 
I should judge that it would arise from a slight mineral deposit 
incrusting the surface of the fossil; but if, as Gumbel asserts, one 
or two layers of cellular structtiire are shown in thin sections of 
this surface-layer, they would, to me, rather indicate the presence 
of an incrusting coral or polyzoan than, as Giimbel supposes, an 
original outer covering of a coriaceous or keratose consistency. No 
mention of this carbonaceous layer has been made by any other 
independent observer, or in any examples from other localities than 
Ober-Kunzendorf, and Giimbel himself does not attach any particular 
importance to it. In the upper calcite layer of Gimbel, which is 
the equivalent of my summit-plates and the ectorhin of Billings, 
this author does not appear to have observed more than radiating 
fibrous grooves, probably of mineral origin. The lower calcitic 
layer of Giimbel’s Kalkplattchen comprises the horizontal spicular 
rays, and I agree with Billings in regarding these structures, which 
* Beitr. p. 21. 
