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, 1 and as having been derived from Cretaceous strata. A similarity in their 

 canal-structures to those of true Siphonice 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 2 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 3 and by Roemer, with the Hexactinellidge,* from the 

 supposed six-rayed character of the spicules ; but after that Dr. R. Martin 5 had 

 pointed out the variable number of the rays in the spicules, Zittel 6 removed the 

 genus to the Anomocladina family of the Lithistidae, to which it is naturally allied 

 both in genera] 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. There 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 7 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. 



* ' Lethaea pal.,' p. 307. 



5 " Untersuchung iiber die Organisation von Astylospongia," ' Archiv des Ver. d. Freunde d. 

 Naturgesch. in Mecklenburg,' Jahrg., xxxi, 1877. 



6 " Ueber Astylospongidae und Anomocladina," ' Neues Jahrb.,' 1884, Bd. ii, p. 75. 



7 See Sollas, " On Vetulina stalactites" (O. S.), ' Proc. Roy. Irish Acad.,' 2 ser., vol. iv, p. 486. 



