552 THE AMERICAN NATURALIST. (VoL: XXXIX. 
ters. Such a decided change in method of growth, with an 
increase in the thickness of the shell without corresponding 
increase in the number of chambers, may be compared in a 
general way to the building up of tissue in the old age of 
Brachiopoda and Mollusca where there is no increase in size 
of the living chamber but frequently a decrease. It may be 
more closely compared to similar conditions which appear in 
the Gastropoda for example in the several Eocene species of 
Calyptraphorus where by reflection of the mantle in later 
growth a callous develops which hides the previous sculpturing. 
RETURN TO SIMPLER CONDITIONS. 
Dimorphina.— In this genus, the development of which has 
already been noted, there is a return to a straight uniserial 
growth in the later chambers. This was expressed by the 
formula E-NFPINIL Ik is a true return to the Nodo- 
sarian and Lagena characters seen in the progressive develop- 
ment of the first two chambers. The biserial Polymorphine 
character in Dimorphina is accelerated so that it may appear 
in but two chambers including the third and fourth 
and a return to the uniserial condition is then taken 
on in the fifth. This return to the simpler Nodo- 
sarian condition, seen in the young of the same in- 
dividual and again as the acme of development in an 
ancestral type, Nodosaria, can be regarded only asa 
truly senescent character and the genus Dimorphina 
a senescent one derived from Polymorphina. 
Dentalinopsis — In this genus (Fig. 25), the early 
: chambers are uniserial like those of Nodosaria. At 
iih ues Bst they are circular in cross section but soon 
Vidi Tm become triangular or polygonal. At this point the 
» inge s Species which have this character as their highest 
last-formeg development are classed under the genus Rhab- 
= (After dogonium which is characterized by angular sec- 
tion in the adult. The formula for Rhabdogonium 
would be L+N4R 

