ASTYLOSPONGIA. 113 
cloacal depression or freely at the summit. The spicular structure is a firm 
resistant mesh-work composed of spicules with solid rounded nodes or centres, 
from which from six to nine straight arms radiate in different directions. The 
spicular rays terminate in branched and slightly expanded processes, which are 
closely apposed to the nodes of adjoining spicules to form the skeletal meshwork ; 
in some cases, also, the rays meet where no centres exist, and their extremities 
partially interlock together and form a pseudo-node by their union. 
The Sponges of this genus were originally regarded as belonging to the genus 
Siphonia,' and as having been derived from Cretaceous strata. A similarity in their 
canal-structures to those of true Siphoniw supported this belief, and their actual 
occurrence in the Drift deposits of Northern Germany mingled with Chalk Sponges 
was accepted as a confirmation of their coexistence in the Cretaceous strata. The 
subsequent discovery by F. Roemer’ of the same forms in unquestionably Silurian 
strata in North America led to a recognition of their true position. They were 
then placed, both by Zittel’ and by Roemer, with the Hexactinellide,* from the 
supposed six-rayed character of the spicules; but after that Dr. R. Martin® had 
pointed out the variable number of the rays in the spicules, Zittel’ removed the 
genus to the Anomocladina family of the Lithistidee, to which it is naturally allied 
both in general form, in its canal-systems and its spicular structure.  Zittel, 
however, now regards the elementary spicules of the genus as simple rods with 
branching extremities, which by their union together form the nodes. here is 
some difficulty in determining the character of the elementary spicules, since in 
no instance at present have they been found detached, whilst in the connected 
skeletal mesh the union is so intimate that their elementary characters are 
concealed. In the recent’ Vetulina stalactites, Os. Schmidt, which has been placed 
by Zittel in the same family as Astylospongia, the elementary spicules clearly 
consist of rays projecting from central nodes, and there is reason to conclude 
that the spicular elements of Astylospongia were similarly constituted. 
The species of this genus are limited to Ordovician and Silurian strata, 
principally the latter. They are comparatively abundant in North America and 
in the Silurian districts of the Baltic, but with the single exception of the form 
mentioned below from Caradoc strata, the genus is not otherwise represented in 
this country. 
1 ¢ Petref. Germ.,’ vol. i, p. 17. 
2 © Die silurische Fauna d. westl. Tenn.,’ p. 5. 
3 ‘Studien I,’ p. 44. 
4 * Lethea pal.,’ p. 307. 
5 “Untersuchung iiber die Organisation von Astylospongia,” ‘Archiv des Ver. d. Freunde d. 
Naturgesch. in Mecklenburg,’ Jahrg., xxxi, 1877. 
6 “ Ueber Astylospongide und Anomocladina,”’ ‘ Neues Jahrb.,’ 1884, Bd. ii, p. 75. 
7 See Sollas, “‘ On Vetulina stalactites” (O. 8.), ‘ Proc. Roy. Irish Acad.,’ 2 ser., vol. iv, p. 486. 
