546 THE AMERICAN NATURALIST. [Vor. XXXIX. 
escent or regressive development. Such forms as that figured 
and the majority of species usually classed under Marginulina 
are not intermediate between Nodosaria and Cristellaria as is 
commonly stated,- but are truly senescent species derived from 
Cristellaria and resulting from acceleration of development and 
the shoving back of the Cristellarian character to the early 
stages in development. 
Polymorphina.—The proloculum or initial chamber in Polymor- 
phina as shown in Fig. 15, is here also simple, like Lagena, as in 
the previous genera. The second chamber (Pl. ı, Fig. 15) is 
added in the same manner as in Nodo- 
saria and in the second stage in all its : 
visible characters Polymorphina appar- 
ently belongs to that genus. The addi- 
tion of the third chamber (Pl. ı, Fig. 16) 
initiates a new character in the line of 
development. It continues back over 
the preceding one to the proloculum as 
in Cristellaria but is added in a differ- 
ent manner being so placed that the 
aperture faces the line of the median 

Fic. ı5.— Section through 
axis. This generic character may be median plane of Polymor- 
represented by P and the formula for y amemus € 
the genus may be expressed as follows : 
Polymorphina = L + N + P. The fourth chamber also extends 
back to the initial chamber (Pl. 1, Fig. 17) but is added on the 
opposite side from the third with its aperture facing and 
occluding that of the third, and this character continues 
throughout further growth to the adult. This is then a pro- 
gressive form and after its Nodosarian second stage, starts a 
new line of development which at once places it in a different 
group from Cristellaria and Marginulina. 
Dimorphina. — In this genus the early stages up to the fourth 
chamber (Pl. ı, Fig. 20,) are exactly as in Polymorphina and 
may be represented by the same formula, L -- N +P. If the 
animal had died at this stage it would be described as Poly- 
morphina. After this stage, however, a quite different mode 
of growth is assumed. Beginning with the fifth chamber, suc- 
