HALVSITID.-E AND TETRADIID.E. 



233 



cite. Hence we could hardly hope to detect mural pores, even 

 if such structures had at one time existed. For the same reason 

 it must remain doubtful whether or not the walls of contiguous 

 corallites are actually amalgamated with one another. Mr 

 Etheridge and myself arrived at the conclusion that the walls 

 were really double (as in Favosites) ; but we based this opinion 

 chiefly upon the apparent exposure of the exterior of the 

 tubes in some fractured specimens, and I have subsequently 

 seen reason to think that the appearances in question are not 

 constant. I still think that the walls are really double ; but as 

 the microscopic evidence is not conclusive, I can only leave the 

 point an open one. 



The most characteristic feature in the corallum of Tctraduun, 

 Avhich separates it, even to superficial inspection, from that of 

 a massive ChcBtetes, is the conspicuously cruciform or petaloid 

 form of the calices or of transverse sections of the corallites 

 (fig. 33, b). This petaloid form is due to the presence of four 

 delicate lamellar septa, which look as if formed by inward fold- 



Fig. 33, — A, Fragment of a large corallum of Tdi-adiiiin minus, Safford, from the Cincinnati 

 Group of North America, of the natural size ; B, Transverse section of the same, enlarged 

 ten times, showing the petaloid form of the tubes and the short septa ; c, Vertical section 

 of the same similarly enlarged, showing the tabulie. 



ino-s of the wall, and which extend for a short distance only 

 towards the centre of the visceral chamber. Normally, and 

 most generally, four of these septal lamina; are present, but 

 their number is not constant, and varies from one to five. 

 The only other special point brought out by sections is, that 



