22 TJie American Geologist. July, i89i> 
results. The section of the corallum is subquadratic; the 
four corallites are of equal size, and the walls are still con- 
siderabl}' thicker than those of later stages. The young 
corallites have a symmetrical section, because the lobes at 
the angles of the square are, notwithstanding their being 
oldest, smaller that the others, as if the growth had been 
checked by the thick common wall of the colon}\ (See 
figs. 3- 4, 8.) 
The four secondary corallites complete their fission in- 
dependently of each other. Quite often one corallite will 
divide long before the others and a corallum with seven cor- 
allites will result (fig. 5). This stage with seven corallites 
is as common as the next stages with loor 13 corallites (figs. 
6 and 7), where 2 resp. 3 corallites have divided. In the 
majority of the cases, however, all four secondary corallites 
divide harmoniously. Sections with 16 equally developed 
corallites (fig. 8) are therefore hardly ever missed on a chip 
of the Tetradium-h^dLXing bank. The fission proceeds regu- 
larly and coralla with 19 and 22 corallites still show distinct- 
ly their derivation and the former arrangement of the 
mother corallites, while exceeding the last-mentioned num- 
ber the general struggle of the young corallites for space 
brings about a general displacement of the latter, and ren- 
ders it very difficult, and often impossible, to find which 
corallites belong together. 
Subquadratic or circular sections of coralla with more than 
16 corallites are, however, by far not so common as flat, rib- 
bon-like coralla which consist of a few or even of only one row 
of corallites. The smallest number of corallites in such flat 
coralla has been found to be 8, arranged in two rows. Coralla 
of this composition are extremely common. The tracing of 
their development by successive sections brought out the 
fact, that they are the result of a fission of the regularly de- 
veloped subcircular coralla, which fission begins when the 
latter have reached the stage with 16 corallites. Fig. 11 repre- 
sents a corallum of 16 corallites in the act of fission; fig. 12 a 
later stage of the same. 
After the fission of the whole corallum, a multiplication by 
fission of the corallites, on both ends of the bands, sets in, re- 
sulting in l^ranches with shoe sole-like (fig. 13) or dumb bell- 
