304 M. F. Dreyer on the 
a primary axis is not the only thing, but their fundamental 
form undergoes further differentiations. In the first place a 
difference of the transverse axes makes itself felt in such a 
way that a long and a short transverse axis may be recog- 
nized, these being perpendicular to each other and to the 
principal axis. The result represents the fundamental form 
of the amphitect pyramid (Hickel) ; the forms belonging here 
are lenticularly flattened laterally, 7. e. parallel to the primary 
axis. In such Rhizopod shells, moreover, the pylom is fre- 
quently no longer round, but drawn out in the form of a slit ; 
any spines present at the aboral pole are generally orientated 
in the direction of the longer cross-axis ; sometimes the peri- 
phery of the monaxon-lenticular shell is keeled. Such more 
or less distinctly amphitect-pyramidal promorphs occur in 
Hyalosphenia, Quadrula, D¢flugia, Euglypha, Gromia, 
Lagena (Fissurina, Rss.), and Lingulina, nm some pylomatic 
Spumiellaria *, in various Nassellaria, and throughout in the 
Pheodarian family Challengerida. 
A further step towards higher differentiation is the tran- 
sition to the eudipleural (bilaterally symmetrical) fundamental 
form, which may start either from amphitect or from simply 
monaxonic forms. This takes place in general in conse- 
quence of an elongation of the pylom (which in monaxonic 
and amphitect Rhizopoda is situated at one pole of the primary 
axis, and, indeed, directly perpendicularly beneath the apical 
pole of the shell) forward or backward, by which means a 
front and back and right and left become distinguishable. It 
is interesting to mark the agreement of this process with the 
transformation of the primary form in the hypothetical deve- 
lopment of the Turbellaria from Ctenophora (A. Lang). The 
Ctenophora and earliest Turbellaria are perfectly amphitect in 
structure ; the mouth is placed in the middle of the underside 
perpendicularly beneath the apical pole of the body, front and 
back, right and left are not yet distinguishable, and this dis- 
tinction is only produced by elongation of the mouth forward 
or backward, which occurs in most Turbellaria (Polyclada), 
and by which the eudipleural fundamental form is given. 
Moreover, many Rhizopoda become eudipleural by a corre- 
sponding arrangement of the oral and aboral radial appendi- 
cular structures or by a bending round of the apertural neck 
of the shell. Eudipleural development of the shell occurs in 
Difflugia, Trinema, Cyphoderia, Campascus, Lieberkiihnia, 
Microgromia, Platoum, Plectophrys, and Lecythium, in many 
* The pylomatic Discoidea and Larcoidea are, however, to be excepted 
from this series of phenomena. For further details upon this point see 
my ‘ Radiolarienstudien,’ Heft i. pp. 98, 99, 
