DR. G. J. HINDE ON RECEPTACULITID®. 831 
in others it is absent, and only the plate remains. Now these scale- 
like spicules are, in reality, modifications of three of the four divergent 
rays of the normal lithistid spicule, and they vary as much from the 
usual development of these rays as do the summit-plates in Recepta- 
culites from the normal sixth ray of the hexactinellid spicule. 
These lithistid surface-spicules show at least the possibility that 
under special conditions the normal cylindrical rays may develop 
into horizontal plates ; and if this is the case in lithistid sponges, 
there is reason to supose that a similar change might take place in 
hexactinellids as well. But though it may be conceded.that no close 
resemblance exists between the summit spicular plates of the Recep- 
taculitide, and the modified sixth ray of the normal hexactinellids, 
yet this difference is insufficient in my opinion to outweigh the close 
similarity which occurs in other rays of the spicules. 
In addition to similarity of form, the disposition of the spicules in 
the Receptaculitidee closely resembles that in some normal hexacti- 
nellid sponges. ‘This is most strikingly shown by a comparison with 
the Cretaceous genus Cincliderma, Hinde *, in which there is a sur- 
face-layer of five-rayed spicules; four of the rays are so arranged 
horizontally that their extremities overlap each other, and form 
definite rectangular interspaces, of a similar character to those 
which are produced by the horizontal rays in [schadites; whilst the 
fifth ray projects invariably in the same manner as the vertical spi- 
cular ray in the latter genus. In the Silurian hexactinellid genus 
Plectoderma, Hindev, there is also a surface-layer composed of spi- 
cules with four horizontal rays and entering vertical ray, but there is 
not the definite quadrate arrangement present in Jschadites. In the 
Cambrian hexactinellid genus Protospongia, Salt.t, the surface- 
squares are formed also by the four horizontal rays, and in this genus 
the fifth ray of the spicule is not developed, thus resembling Sphero- 
spongia. ‘The resemblance of the surface of this sponge to that of 
Receptaculites induced Giimbel to place it in the same group with the 
latter. The Devonian genus Dictyophyton, Hall, has also the same 
regular arrangement of the surface into quadrate interspaces, and 
though the spicules in this genus have not yet been discovered, there 
can hardly be a doubt that they were similar to those of Proto- 
spongia and Plectoderma. It is not without significance that, both 
by M*Coy and by Ferdinand Romer, examples of Dictyophyton have 
been placed in the genus TVetragonis, Hichwald, which, as we have 
seen, is synonymous with Jschadites, Murch. 
With respect to the relative dimensions of the spicules of the Re- 
ceptaculitide, and those of recognized hexactinellid sponges, it may 
be stated that though in some of the species of Receptaculites 
itself, the individual spicules are considerably larger than in 
any known sponge, yet there is not so great a disproportion as to 
render a comparison fanciful or extravagant. Thus, in &. arcticus, 
Etheridge, the largest form of the family known, the spicules are 
from 10 to 18 millim. in length, and about 3 millim. in thickness; 
* Cat. Foss. Sp. pl. 28. f. 1 a—-d. 
ft ddlty shit. t Id. pl. 28. f. 2. 
