8 GENERAL REMARKS. 
lime, excepting in the older parts. The corallite-wall is very perforate and costate externally; 
the costz often project as trabeculz. The wall of young corallites consists of only one layer 
of mural tissue, and is provided externally with plain or finely serrate costz for some distance 
down. In older corallites spimules are always present on the free edges of the coste, and 
these are largest round the bases of the corallites, where they form a basis of lax tissue which 
is the first stage in exogenous growth, and occasionally assists im the production of buds. 
The formation of mural tissue takes place in the following manner :—The spinules act as 
props on which a porous layer of new tissue is formed in such a manner that the old wall 
forms the floor of a chamber, and the spinose costz the supports of the new roof. In old and 
in axial corallites the costze assist in the formation of a new layer of tissue by sending out 
lateral spinules from the free edges. In transverse sections of corallites of moderate thickness 
the wall is seen to consist of “ concentric circles of thin calcareous structure .. . separated by 
radiating pillars; the circles having been, in turn, outside walls and the radii either spinules 
or cost.” If the corallite is old the first, second, or third circles of tissue, next to the 
septal cavity, are dense. The costz are imperforate, and as a rule the septa also. Buds from 
a radial corallite are formed from trabeculz which arch over and form a low hood. The 
costee next appear on the outer surface of the hood, and after elongation the septa appear ; 
these arise first as linear series of spines directed inwards. No communication between the 
cavities of the bud and axial corallite exists, “ except in a very indirect manner and through 
the medium of the dermal structures. Budding takes place remote from the calicular 
margin, and may arise from sclerenchyma remote from the wall of the corallite.” 
In the second species described, probably a variety of M. cytherea, Dana, from Mada- 
gascar, there are numerous immersed corallites. These are closely arranged, and their walls 
are well defined in longitudinal section, and well-developed trabeculz pass completely across 
the polyp-cavities. Between neighbouring corallites the connecting structures are in 
successive layers or storeys of laminz separated by rows of small, irregular, and short pillars. 
On the upper surface between the immersed corallites numerous minute, broad-based, sharp- 
pointed spinules occur, arising from a perforated calcareous lamina; this is precisely the 
condition of the successive layers below the surface, and new layers are evidently here added 
in the same manner as on the corallite-wall. 
The third species described is referred by Duncan to M. granulosa, Kd. & H., but judging 
from the description, and from small carmine-stained fragments which I believe to be the 
material investigated, I am inclined to think it more nearly related to MW. secale, Studer. In 
any case the species serves to illustrate the structure of slow-growing varieties in which the 
corallum is compact. The surface is here very dense, and consists of a stout lamina with 
very few perforations, and is clothed with knobbed spinules in place of costz. In transverse 
section concentric laminz, as in other cases, are separated by radially disposed bars which 
represent former spimules. In this case, however, the laminz are much thicker and denser 
and are closer together. Exceedingly narrow tubes lead from the polyp-cavities to the surface, 
but their apertures are difficult to detect, as they are surrounded by the bases of spines. 
Little need at present be added to the results of Duncan. Evidently the condition of the 
