334 Dr. H. A. Nicholson on the Minute 



which would almost universally be referred to the corals, and 

 which, at any rate, are very unlike the ordinary forms of 

 Polyzoa. Thus it is well known that various species of 

 Favosites (such as F. Forbesi, Ed. and H., var. tuberosa, 

 Rominger, F. turbinate, Billings, and F. clausa, Rominger) 

 are liable to have the mouths of the corallites closed by a 

 calcareous pellicle, which may cover a large part of the sur- 

 face of the colony. 



It only remains to add, with regard to the general external 

 characters of H. neozelanica, that the mouths of the tubes, 

 even when fully exposed by maceration in sea-water (as in 

 fig. 1, B), do not appear to show any signs of radiating spines 

 ("septa"), though, as will be subsequently seen, such really 

 exist in the interior of the cells. It may also be noted, as 

 compared with any ordinary Monticuliporid, that though the 

 skeleton is clearly dimorphic, in the sense that it is made up 

 of two sets of tubes, the difference between the large tubes 

 (zocecia) and the small ones (cancelli) is small and sometimes 

 hardly recognizable. The cancelli, in fact, are often nearly 

 or quite as large as the proper zocecia (see fig. 1, B and C) ; 

 and the chief distinction between them rests upon the gene- 

 rally more clearly angular shape of the former, and upon 

 their mouths not being at all raised above the general surface. 



The minute Structure of the Skeleton o/Heteropora 

 neozelanica, Busk. 



The skeleton of H. neozelanica, as of the species of Hetero- 

 pora generally, is ramose or dendroid ; and the branches 

 resemble those of many similarly shaped corals in being com- 

 posed of fasciculate tubes which are nearly vertical in the 

 axis of the branch, but ultimately bend outwards to reach the 

 surface. We can thus divide each branch into an axial and 

 a peripheral or cortical portion ; and not only do the tubes in 

 these two portions of their course differ in direction , but they 

 are markedly different (as we shall see) in their actual struc- 

 ture. That is to say, the structure of any particular tube is 

 exceedingly different, according as we examine it in the axial 

 or in the cortical part of its course. However, it is in 

 the cortical portion of the skeleton alone, or almost alone, 

 that the interstitial tubes are developed, the axis con- 

 sisting wholly, or almost wholly, of the proper zocecia. We 

 cannot, therefore, arrive at a proper understanding of the true 

 structure of Heteropora (or of any similarly constructed skele- 

 ton) without making three distinct sections, viz. : — one parallel 

 to the surface and just below it, which we may call tangential, 

 and which is the most important of all, as giving us the cross 



