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 

 tlie 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 

 in m.any Thalamophora. This embracing process occurs parti- 

 cularly typically in the Miliolida, and, indeed, we may here 

 recognize a gradual increase from Cornuspira and Spirolocii- 

 lina, in which all the whorls lie 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 

 occurs 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 w^ay that the Thalamophoran shells 

 produced by general embracing may be compared with the 

 concentric sphere-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, Molluscau and Thalamophoran shells 



* See ' Pylombildungeu/ pp. 112, 113, and p. 101, note 1. 



