THECID.^ AND HELIOPORID^. 237 



and obtuse lamellar septa, and communicate directly with one 

 another by means of tortuous horizontal channels passing from 

 one to another. No columella is present. 



Obs. — Owing to the great density of its tissues, the diffi- 

 culties in the way of making a thorough examination of the 

 structure of Thccia by means of the microscope ^ are unusually 

 great, and it has been only by means of a large series of thin 

 sections that I have been enabled to come to any satisfactory 

 conclusions as to its minute anatomy, while there still remain 

 some points of importance which I have found it impossible to 

 clear up. The corallum in the type-species, T. Swindernana, 

 Goldf. sp., has the form of a laminar, usually discoid expansion, 

 attached by the centre of its base to some foreign body, and 

 having the whole of the lower surface covered by a concentri- 

 cally striated and wrinkled epitheca. The upper surface carries 

 the calices, and when examined macroscopically, even by means 

 of a lens, usually shows nothing but the stellate apertures of 

 the larger corallites, which are from a third of a line to half a 

 line in diameter, and are separated by what appears to be an 

 equal width of dense interstitial tissue. This interstitial tissue 

 is marked superficially by numerous minute, radiating, often 

 vermicular grooves, which extend from each calice to the neigh- 

 bouring ones (PI. XI., fig. 2), and it was regarded by Milne- 

 Edwards and Haime as being of the nature of a " spurious 

 ccenenchyma, resulting from the intimate union of the costae " 

 (Brit. Foss. Corals, p. 278). As there are no costee in Thecia, 

 Milne- Edwards and Haime were, of course, in error in suppos- 

 ing this interstitial tissue to be formed by the coalescence of 

 structures of this nature ; but in the sense in which they used 

 the term " ccenenchyma," they were undoubtedly right in 

 supposing that they were dealing here with a " coenenchymal " 

 tissue. In other words, there can be no doubt as to the identity 

 of the interstitial tissue of Thecia with the so-called " ccenen- 



1 Though in the habit of preparing all my own sections for the microscope, I was 

 compelled in this instance to have resort to the skill of Mr F. G. Cuttell; and I have 

 therefore had the advantage of probably the most beautiful sections of this difficult 

 form which could be prepared at the present day. 



