60 G. E. ANDERSON 
of a horseshoe, but in more specialized individuals a bridge span- 
ning the cardinal fossula gives the inner wall a circular outline and 
completely separates the inner circular and the outer annular areas. 
In the Acervularia type* the supposed inner wall is much thicker 
than the septa and in size about one-half the diameter of the corallite, 
so that even the tertiary septa take part in its formation and these as 
well as the secondary septa may extend into the inner central area. The 
supposed inner wall thus formed is not of uniform thickness, being 
thicker at the intersection of the septa: in some individuals the 
thickening from each septum is not sufficient to be in contact with the 
thickening from the neighboring septum, thus giving a number of 
openings in the wall. This is readily accounted for when it is seen 
that the apparent wall is formed by the lateral thickening of the septa 
at or near the end of the tertiary septa. When sufficient thickening 
of the septa occurs, they will be brought in contact, giving the appear- 
ance of an inner wall. It is evident that the supposed inner wall of 
this type is rather of the nature of a 
pseudotheca, the portion of the septa 
between. the region of the septal thicken- 
ing and the outer wall corresponding to 
the costae, thus differing greatly from the 
corals containing a true inner wall. 
We may now inquire into the nature 
of the inner wall itself as present in such 
Fic. 1.—Cross-section of a % S€Nus as Craspedophy llum. The 
young corallite of Craspedo- Devonic species, Craspedophyllum sub- 
phyllum subcaespitosum Diam- ges pitosum, from the Hamilton of Thed- 
eter ann. a. ‘The alar septa. ford, Ontario, is here chosen, as it is one 
c. The cardinal septum. D d y 
Showing the early grouping of Which is especially distinguished as show- 
the septa. Four openings are jing the typical structure of the true inner 
shown but the normal condi- wall 
tion is three, the two alar and aie 
the one at the cardinal fossula. The mode of origin and development 
can be understood by comparing a series 
of figures (Figs. 1-5) representing the development of the inner wall 
1 This refers to description of the type of Acervularia. In some specimens of 
Acervularia in this country there is no lateral thickening of the septa, and hence no 
indication of the supposed inner wall. 
