42 J. E. DUERDEN. 



The ontogeny of the various forms shows that the shorter 

 septa associated with the fossulre cannot be considered as repre- 

 senting a cycle or cycles different from the others. Polycycleism, 

 as we know it in the hexacorallids, does not occur in the tetra- 

 corallids ; however many septa may be developed there are only 

 two cycles represented, the inner larger septa being entosepta 

 while the outer shorter are exosepta. The entosepta include 

 the six primary protosepta arid the later metasepta, some mem- 

 bers of the former being at times larger than the rest (Anisophyl- 

 Iniii). In Menophyllum the metasepta formed within the middle 

 and ventral chambers differ greatly in size, those in the two ventral 

 chambers being the larger. In this genus three fossulae are said 

 to occur, the two lateral situated as before on the dorsal aspect 

 of the alar septa ; the alar septa are not, as is generally assumed, 

 included within the fossula itself. 



As stated in the introduction, a fourth or dorsal-directive fos- 

 sula is sometimes present in rugose corals, but this occurs very 

 rarely, Oinphyma being given as an example. Among a collec- 

 tion of ZapJircntis coinprcssa, E. & H., from Spergen Hill, 

 Indiana, I have found several in which there is in the mature 

 corallite a conspicuous fossula towards both the dorsal and 

 ventral extremities of the principal axis, the ventral fossula being 

 the more pronounced. In the majority of specimens, however, 

 there is no hint of the dorsal pit. The dorsal fossula would 

 appear to have no developmental significance, since the septal 

 arrangement is there the same as in other species of this genus ; 

 rather, it would seem to correspond with some peculiarity of the 

 adult stomodaeum, perhaps with the longer continuation of its 

 dorsal extremity. 



RADIAL AND BILATERAL SYMMETRY. 



A comparison of rugose corals having bilateral symmetry 

 with those in which all the septa are radially arranged introduces 

 considerations with regard to symmetry in corals generally. 

 Mature corals and actinians, like most ccelenterates, are char- 

 acterized by a more or less perfectly radial symmetry as regards 

 their mesenteries, tentacles, and septa, yet throughout the course 

 of their development these organs follow a decided bilateral 

 method, both in the earlier and later stages. 



