300 M. F.. Dreyer on the 
The last of them, which bears the furca, is also unsymmetri- 
cally developed. In correspondence with the relative sizes of 
the two halves of the furca, the left side of the segment in 
question is considerably broader than the right. The serra- 
tion characteristic of the furca of this species terminates regu- 
larly on each side just above the lateral seta, but on the right 
side it commences, not as on the left immediately beneath the 
hinder margin of the last segment, but somewhat further back. 
This malformation may be original, but is more probably pro- 
duced by a subsequent accident (perhaps in change of skin). 
In the reproduction of the lost part ot the furca it was not 
again completed of the normal size; but in this way certainly 
the displacement of the above-mentioned seta is not easy to 
explain. 
XL.— Considerations on the Structure of Rhizopod Shells. 
By FRreprico DREYER *. 
In the course of my investigations upon Rhizopoda, and 
especially upon the Radiolaria, various considerations of a 
general character have impressed themselves upon me. In 
part similar ideas have been already touched upon by previous 
authors and occur scattered in the most various parts of the 
copious literature; several points to be referred to in what 
follows I have already incorporated with the special investi- 
gations in the first part of my ‘ Radiolarienstudien’ +; never- 
theless I regard it as a not unprofitable task to reproduce in 
the following pages in a connected form the complete train of 
thought of my considerations upon the structure of the Rhizo- 
pod shells, as I hope that it will be of interest even for many 
who do not occupy themselves specially with the Protistan 
group in question. 
Even on a superficial consideration of the enormous num- 
ber of forms of the Rhizopoda we may recognize in them an 
essential difference in the general habit of the shell, and, in 
accordance therewith, distinguish two groups of forms. One 
* Translated from the ‘ Biologisches Centralblatt,’ Bd. ix. pp. 383-352 
(1st August, 1889). ’ ; ‘ 
+ F. Dreyer, ‘ Morphologische MRadiolarienstudien,’ Heft IL “ Die 
Pylombijdungen in vergleichend-anatomischer und entwicklungsgeschicht- 
licher Beziehung bei Radiolarien und Protisten tiberhaupt, nebst System 
und Beschreibung neuer und der bis jetzt bekannten pylomatischen Spu- 
mellarien ” (Jena, 1889). 
