THE VOYAGE OE H.M.S. CHALLENGER. 
viii 
descriptive portions of the text might have been very much extended, but the intention 
has been to limit them to cases in which the identity of the forms referred to is unmis- 
takable ; and they are sufficient to indicate the difficulties that have been created by the 
introduction of useless names, and the need that exists for the revision of the generic 
and specific nomenclature. 
Dimorphism. — The terms “ dimorphism ” and “ dimorphous,” which are of frequent 
occurrence in works on the Foraminifera, have been used in connection with two distinct 
phenomena, and appear to require a few words of explanation. 
In their original sense these terms refer to shells exhibiting two modes of 
growth. Amongst the Lagenid^e, for example, it often happens that two allied genera, 
differing chiefly in the arrangement of their segments, are connected by an intermediate 
group which partakes to a greater or less degree of the morphological characters of both. 
Thus the characters of Cristellaria and Frondicularia are displayed by the dimorphous 
type Flabellina, the early chambers of which are planospiral, whilst the later segments 
are embracing and form a complanate rectilinear series. In the same way, Dimorphina 
presents the characters of Polymorphina and Nodosaria, the early segments forming an 
elevated spire, the later ones a single row, either curved or straight. It is sometimes 
d iffi cult to say to which genus the intermediate forms more properly belong (perhaps no 
general rule applies), and it has been usual to treat them collectively as distinct genera and 
subgenera. Other examples of dimorphism are found amongst the Textularida;, but 
under somewhat different conditions from those which have been cited ; and though mani- 
fested in a variety of ways it does not result in the formation of the same sort of inter- 
mediate groups. The dimorphism of the Textularida depends, generally speaking, upon 
the tendency of the normally multiserial test to assume a simpler arrangement. Thus, a 
test biserial at its commencement becomes uniserial in its later growth ( Bigenerinci ); 
one triserial to begin with becomes biserial ( Gaudryina ), or uniserial ( Clavulina ), and 
so on. In rare instances, examples of “ trimorphism ” may be met with, as in one of the 
elongated varieties of Spiroplecta, the segments of which are arranged planospirally 
at the commencement, subsequently as an alternating binary series, and finally in 
a single line. 
“Dimorphism” therefore, as understood by d’Orbigny, Parker and Jones, Carpenter, 
and others, implies the existence of two modes of growth in the individual shell. 
The recent interesting researches of MM. Munier-Chalmas and Schlumberger on the 
“ Dimorphism of the Foraminifera ” refer to quite another feature of the Order, namely, to 
the occurrence of the same species in two distinct forms. The late Dr. Philippe de la 
Harpe was the first to observe that there were certain Nummulites which were usually, 
if not invariably, distributed in pairs ; and that these pairs, whilst agreeing as to general 
external characters, differed in point of size ; furthermore that the internal structure of the 
