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 Kadiolaria, and, 

 indeed, in Sph^roidea, 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 outw^ards 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 so on. 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 Nodosariie, or curved, as in 

 Dentalina, or rolled into a spiral (e. g. CristeUana) , like the 

 shells of the Nautiloidea and Ammonites, only in the latter 

 the soft body is exclusively iu the last or youngest chamber, 



