318 M. F. Dreyer on the 
the smallest possible surface to external mechanical attacks ; 
it is just otherwise with the products of the terminal growth- 
process, in which the different chambers are arranged one 
after the other in the form of a longer or shorter chain. 
Leaving out of consideration that such a series of chambers 
of considerable length is very obstructive to locomotion, it is 
comparatively very frangible, and from the statical or 
mechanical point of view disadvantageous. The Thalamo- 
phora avoid these disadvantages of the terminal growth and 
combine the advantages of the concentric shell-system with 
the terminal growth by generally not leaving their series of 
chambers in an extended state, but rolling them up spirally in 
the majority of the forms. As a further carrying out of the 
spiral convolution we must regard the reciprocal embracing of 
the chambers which occurs'in a more or less marked manner 
inmany Thalamophora. ‘This embracing process occurs parti- 
cularly typically in the Miliolida, and, indeed, we may here 
recognize a gradual increase from Cornuspira and Spirolocu- 
lina, in which all the whorls he freely exposed, through 
Quinqueloculina, Triloculina, and Biloculina to Uniloculina. 
In the last-mentioned genus the process has attained its 
highest point, for here only the youngest chamber is freely 
~ exposed externally, while all the preceding chambers are 
completely enclosed by it. Here consequently exactly the 
same final result is attained as in the concentrically formed 
shell-systems of the Radiolaria, although in a quite different 
way. If the embracing of the chambers takes place only in 
one plane, this leads to the so-called cyclical growth, such as 
oceurs in Orbiculina, Orbitolites, Cycloclypeus, and similar 
forms. ‘There is thus produced within the terminal growth- 
type an apparently concentric growth, just as a number of 
discoid Radiolaria appear to grow spirally, that is terminally. 
These apparent exceptions to the rule above established, that 
no Rhizopod shell can change its growth-type, always turn 
out, however, on closer examination to be secondary conver- 
gences or analogical structures, although certainly sometimes 
deceptive *. In the same way that the Thalamophoran shells 
produced by general embracing may be compared with the 
concentric spuere-systems of the Radiolaria, the cyclical 
Thalamophoran shells represent the concentric ring-systems 
of the Discoid Radiolaria. 
In conclusion may be mentioned the extremely interesting 
and significant fact that, according to the investigations of 
Naumann and v. Moller, Molluscan and Thalamophoran shells 
* See ‘Pylombildungen,’ pp. 112, 113, and p. 191, note 1. 
