SOCIALITY IN THE MADREPORARIA 
243 
gonum in which the assumption of a definite hexagonal shape is 
caused by maximum crowding of calices. 
In the species Lithostrotion canadense, although the calices have 
taken on the hexagonal shape, each column remains distinct, the 
walls of adjacent columns being merely applied to each other. 
In Acervularia and other genera the walls of adjacent columns 
have coalesced and the common wall brings about the closest 
union between polyps attainable without actual confluence. 
Colonies may arise in two ways: by budding and by fission. 
Stolonization is to be considered as a type of budding. These 
two lines of development from solitary corallite to colonial form 
may be illustrated in the Perforata within the single family Eup- 
sammidae by the genera Balanophyllia, Heteropsammia, Lobop- 
sammia, Dendrophyllia and Rhizopsammia. 
The Solitary Phase. Balanophyllia may be taken as a typical 
example of a solitary coral, and this, as well as other solitary 
forms, is often found in pseudo-colonial groups. 
Colonies arising by Fission. An example of a genus that 
has abandoned the solitary form and is in process of becoming 
colonial is Heteropsammia, One species, H. michelini, invariably 
occurs with two calices, partly imbedded in coenenchyma. An- 
other species, H. multilohata, has made further progress, always 
having a greater number of calices than two. A further departure 
along the same line is shown by Lohopsammia, in which there is 
repeated fission, resulting in a cespitose colony. 
Colonies formed by Budding. Balanophyllia forms new in- 
dividuals by budding, as do other solitary corals; and it is easy 
to see that a colony might arise in the first instance by retention 
of a bud. The genus Rhodopsammia shows a primary stage in 
colony formation by retaining lateral buds which in turn may bud, 
and the grand-daughter buds being likewise retained, the result is 
a dendroid colony of a simple type. A more advanced stage is 
illustrated by the genus Dendrophyllia, some species of which on 
the one hand build up by successive gemmations dendroid colonies 
of considerable complexity (Z). nigrescens, etc.) ; and on the other 
hand certain species {D. willeyi, D. diaphana) form cespitose and 
finally massive colonies by lateral or sub-basal budding. Stoloni- 
zation is represented by the genus Rhizopsammia. 
Aporosa. The Aporosa furnish no single family, the limits of 
which are undoubted, to illustrate the two lines of development in 
colony formation, but a fairly complete series may be found 
within the limits of the suborder as a whole. 
