306 M. F. Dreyer on the 
by radial rods, the so-called radial beams. The growth of 
the hollow spheres following upon the first shell in a great 
number of cases (perhaps always?) even proceeds from the 
radial beams, the ends of these, which radiate freely outwards 
as radial spines, emitting a system of lateral apophyses, which 
grow together and complete the next shell. This is the typical 
and original form of the concentric shell-growth ; it occurs, 
like concentric growth in general, only in Radiolaria, and, 
indeed, in Spheroidea, many Prunoidea, the Phacodiscida, 
and the Phractopeltida. This original course undergoes 
modifications by the growth taking place no longer on all 
sides, but instead of this in definite directions. Thus the 
disciform Discoidea grow only in one plane by the addition of 
concentric rings; many Prunoidea only in the direction of 
one axis, as in them a series of dome-shaped segments of 
spheres are added successively at the two poles. Both modi- 
fications, however, may be easily referred to a system of con- 
centric spheres and explained naturally as follows :—That in 
the Discoidea only those parts of the latticed spheres which 
are situated in the plane of growth are developed as rings, 
while in the Prunoidea only the sphere-segments placed at 
the two poles of the principal axis in which growth takes 
place are developed. 
As we have seen, in the concentric growth-type an addition 
of new portions of shell originally takes place uniformly in 
all directions, or in the last-mentioned modified modes of the 
phenomenon at least in more than one direction. In opposi- 
tion to this the shells of the terminal growth-type grow only 
in one direction. Just as the concentric growth-type is asso- 
ciated with the perforate form-type, so is the terminal type of 
growth with the pylomatic form-type. Terminal shell-growth 
takes place in this manner :—The sarcode-body of a pylo- 
matic shell, as soon as the latter becomes too small for it, 
swells forth in part from the pylom, and in front of this forms 
a second shell (here usually called a chamber or joint), which 
opens outwards by a new terminal pylom. In the further 
growth of the soft body this process is repeated again and 
again ; in advance of the pylom of the second chamber a third 
chamber is formed, in front of this a fourth, and soon. In 
this way longer or shorter series of chambers are produced, 
which continue to grow at their extremity, the orificial pole of 
the youngest chamber. ‘The series of chambers is either 
straight, as in Cystoidea and Nodosariz, or curved, as in 
Dentalina, or rolled into a spiral (e. g. Cristellarta), like the 
shells of the Nautiloidea and Ammonites, only in the latter 
the soft body is exclusively in the last or youngest chamber, 
