MADREPORARIA APOROSA. 



VI 



approaches the Fitngida, and it has been commonly regarded as con- 

 stituting a special family of the Aporosa (the Pseudofungidce). 



Another remarkable type is constituted by the genus Holocystis of the 

 Lower Greensand (Cretaceous). In this genus (fig. 150) the corallum is 

 composite, the corallites being connected directly by their walls or by 

 costae. There is a styliform columella, and the septa are developed in 

 three cycles, there being four principal septa of much larger size than 

 the others. The symmetry is thus conspicuously tetrameral, and for 

 this reason Holocystis has generally been regarded as belonging to the 

 Rugosa. The visceral chambers of the corallites are also intersected by 

 tabulae, these structures being likewise abundantly developed in the 

 allied genus Coccophyllum of the Alpine Trias. 



The Silurian genus Stauria, which has been usually referred to the 

 Rugosa ( Tetracoralla), and which has been regarded as the type of 

 the special family of the StauridcE, may also be regarded as belonging to 

 the Astro?ida?, since it differs from the typical Astraeans in little else than 

 its marked tetrameral symmetry. The corallum in Stauria (fig. 151) is 



ws$m 



Fig. 151 — A few calices of Stauria 

 astrceiformis, enlarged, showing the 

 four primary septa forming a four- 

 branched cross. Silurian. (After 

 Milne -Edwards and Haime.) 



Fig. 152.— Transverse section of a single corallite 

 of Stauria astrceifor?nis, from the Silurian of Got- 

 land, enlarged ten times, v, Primordial wall ; s, 

 Secondary layer of stereoplasma, the corresponding 

 layer in the interior of the corallite being shaded. 

 (Original.) 



composite and astraeiform, increase being effected by calicular gemma- 

 tion, and the corallites being connected directly by their walls. There 

 is no columella, but there are four principal septa which form a com- 

 plete cross in each corallite (fig. 152). These four septa divide the vis- 

 ceral chamber into as many quadrants, each of which contains three long 

 and four short septa. The total number of septa is thus thirty-two, six- 

 teen long and sixteen short. The periphery of the visceral chamber is 

 occupied by vesicular dissepimental tissue,' while the central area is 

 traversed by horizontal tabulae. The only known species of this genus 

 is the Stauria astrcziformis of the Wenlock Limestone. 



It would also seem not improbable that the Palaeozoic genus Acer- 

 vularia, properly so called (in so far as based upon Acetvularia ananas, 

 Linn., of the Wenlock Limestone of Gotland), will have to be placed 

 among the Astraida, though this point does not admit of discussion 

 here. On the other hand, many of the forms usually referred to Acer- 



VOL. I. S 



