No. 464.] STAGES IN LAGENIDE. 547 
ceeding chambers are arranged in a linear series and are there- 
fore Nodosarian in character. The formula for the adult would 
be, Dimorphina = L+ N 4- P-- N. Thelast chamber is often 
more or less free and like Lagena. The formula for the com- 
pleted growth including this gerontic character will then be, 
Dimorphina = L 4- N - P --N +L. Dimorphina is therefore 
a senescent genus having as its basis the three progressive char- 
acters of Polymorphina and in senescence has, by retracing in 
inverse order, attained in the last chamber to a condition com- 
parable to that seen in the proloculum, or in other words to 
Lagena. Having completed the cycle, such forms evidently 
have attained completion along certain lines of development 
and no new forms can originate from this senescent type except 
in regard to minor changes in character of ornamentation which 
are recognized as specific and not generic in value. 
SENESCENT CHARACTERS. 
Characters showing typical senescence are frequent among 
the Foraminifera and illustrate in simple terms conditions simi- 
lar to those found in many other groups of the animal kingdom. 
The simplest of these senescent characters is the loss of orna- 
mentation. The ornamentation of this group is shown mainly 
by elevated ridges, by tubercles, or by spines. Examples of the 
loss of these will be given. As found by Hyatt in Cephalopoda, 
the uncoiling of forms, which in their early development were 
close-coiled, is a decided feature of senescence. Such cases are 
seen in their simplest terms among the Foraminifera. As 
shown by Beecher, one of the characters apt to appear in the 
decline of a group is a peculiar spinose or extravagant growth, 
“wild” growth, as it is sometimes called in the Foraminifera. 
Certain senescent genera of Foraminifera show this character 
exceptionally well. One of the surest indications of senescence 
is a return to the simpler conditions seen in the young of the 
ual and in the adults of more primitive forms. Ex- 
same time very simple cases of this are seen 
The Foraminifera as uni- 
same individ 
cellent and at the 
in Dimorphina and in Dentalinopsis. 
