356 Prof. A. E. Verrill on the Affinities of Palceozoic 



contrary, there are either no transverse plates, or else they exist 

 between the radiating lamellae or septa, thus dividing each of 

 the radiating chambers into a series of transverse cavities, which 

 are usually not exactly on the same level in the diflferent cham- 

 bers. At the time when this classification was proposed, the 

 polyps of but few of the " tabulate corals" had been examined, 

 and no characters were drawn from the soft parts. The explana- 

 tion of the transverse septa seems to be, judging from my own 

 dissections and also from analogy with other animals, that they 

 are formed after each discharge of ova 5 the vacuity thus pro- 

 duced, being useless, is cut off from the visceral cavity above it 

 by the formation of a septum. Therefore, if the eggs be dis- 

 charged from all the radiating chambers simultaneously, or if 

 from any other cause the polyp abandons all the chambers simal- 

 taneousiy, it is obvious that a complete septum or transverse 

 plate will be formed across the entire tube ; but if the eggs be 

 discharged at different times from the ovaries occupying the 

 various radiating chambers, the septa formed below them in the 

 different chambers will not be coincident or exactly at the same 

 level in all. It would seem, therefore, that the existence or 

 non-existence of complete transverse plates is simply a matter 

 of periodicity in the discharge of ova. 



We should naturally expect to find such variations in pe- 

 riodicity among the species and genera of many diverse groups ; 

 and this, I think, can easily be shown to be the case. Thus, 

 for example, the genus Coelastrcea, V., an undoubted Astrsean 

 coral, has the septa in all the chambers on the same level, thus 

 forming true tabulse; the genus Alveopora (fig. 1, a), and 



Fig. 1. 



a, a longitudinal section of Alveopora spongiosa, Dana ; b, a vertical view 

 of some of the cells : both much enlarged, copied from Dana's Atlas 

 of the Zoophj^tes of the U. S. Expl. Exp. For the use of this cut I 

 am indebted to Messrs. Dodd and Mead, the publishers of Professor 

 Dana's new work on Corals and Coral Islands, 



