MORPHOLOGY OF THE MADREPORARIA. 99 



sidered as entosepta and exosepta, the formula would be 6, 6, 

 6, 1 8 ; the three first numerals in the latter indicate the entosepta 

 and the last the exosepta. The cyclic formula of a corallite 

 with 40 septa would be 6, 6, 12, 16, and the morphological 

 formula 6, 6, 8, 20. In the first morphological formula 12 of 

 the exosepta will be quaternaries and 6 will be tertiaries ; in the 

 second 16 will be quaternaries and 4 tertiaries. 



Where the relationships of the septa to the mesenteries are 

 clearly known the morphological formula will more nearly 

 express the real value of the septa than the ordinary cyclic 

 formula, the latter has little significance unless the hexameral 

 sequence is fully completed. One is not justified in saying that 

 a cycle is really complete unless all its constituents have the 

 same morphological value, and this is not the case when some 

 are entosepta and some are exosepta. 



A few remarks may here be made concerning the dorso-ventral 

 appearance of the organs in corals generally, and the consequent 

 marked bilaterally of the calice for the time being. 



In palaeontological literature much has been made of the fact 

 that the Palaeozoic rugose corals (Tetracoralld] are bilaterally 

 symmetrical, while most modern corals are radially symmetrical. 

 The results here outlined prove however that modern corals are 

 strongly bilateral in the course of their development, and that it 

 is only when the septa are fully established that an approximate 

 radiality is assumed. In like manner I have found that many 

 rugose corals attain perfect or almost perfect radiality when 

 maturity is reached, though the developmental stages are strongly 

 bilateral. Radiality in the Actinozoa, as compared with bilate- 

 rality, seems to have a more ontogenetic than phylogenetic sig- 

 nificance. Furthermore, beyond the six primary members the 

 septa in the Rugosa are added in a manner altogether different 

 from that in modern hexameral corals, hence the bilaterality of 

 the one group has a different origin from that of the other. The 

 subject of bilaterality in the Rjigosa will be more fully discussed 

 in a later paper. 



In modern corals the bilaterality of the polyp during develop- 

 ment may be looked upon as associated in turn with each cycle 



