THECID^E AND HELIOPORID^E. 247 



when we examine tangential sections of the corallum (PI. XI., 

 fig- S)> we fi n d that the polygonal and comparatively regular 

 tubes of Heliolites are replaced by quite irregular spaces, of 

 very variable size, surrounded for the most part by curved 

 boundaries, which represent the cut edges of the component 

 vesicles of the interstitial tissue. 



With the exception of the imperfectly characterised Plasmo- 

 pora micropora (Goldfuss), E. and H., which is believed to be 

 from the Devonian of the Eifel, all the species of this genus 

 are Silurian in their range. 



Genus PROPORA, Edwards and Haime, 1849. 



(Compt. Rend., t. xxix. p. 262.) 



(PL XI, figs. 3-3 A) 



Gen. Char. Corallum discoidal, hemispherical, or irregularly 

 spheroidal, the lower surface covered with a concentrically- 

 striated epitheca. Corallites of two sizes : the larger ones with 

 distinct walls, which are infolded so as to form twelve short 

 septal ridges, the visceral chamber being intersected by com- 

 paratively remote, complete, approximately horizontal tabulae. 

 Smaller corallites with altogether obsolete walls, these struc- 

 tures being undistinguishably amalgamated with the convex 

 tabulae, so that the narrow interspaces between the larger tubes 

 become filled up with a vesicular tissue formed of lenticular 

 cells. Calices slightly exsert. 



Obs. I am quite disposed to agree with Dr Lindstrom 

 (Ann. Nat Hist, ser. 4, vol. xviii. p. 16) in thinking that 

 Propora can hardly be kept generically distinct from Plas- 

 mopora. The mere fact that the larger corallites are slightly 

 exsert can hardly be regarded as of generic importance ; and 

 an examination of thin sections of Propora tubulata, E. and 

 H., has led me to take a different view of the condition of the 

 septa to that propounded by Edwards and Haime. These 

 high authorities believed that the twelve short septa of the 



