316 M. F. Dreyer on the 



corresponding isomorphous calcareous forms are directly 

 connected, while the most highly developed and most differen- 

 tiated terminal types of the stems are exclusively of calcareous 

 nature. 



This phylogeny of the Thalamophora of Neumayr's har- 

 monizes perfectly with our conception of the significance of 

 the structural material of the Rhizopod shell, and the two 

 theories lend each other a support which must not be under- 

 valued. The lower and lowest forms find the coarse aggluti- 

 nated material quite sufficient for the construction of their 

 simpler shells ; the forms of medium complication already 

 for the most part have recourse to carbonate of lime ; while, 

 finally, the most differentiated types construct their shells 

 exclusively of lime, because this finer and firmer material 

 alone renders possible that complicated structure which could 

 not be carried out with the coarse and less solid agglutinated 

 material. Only in the case of one of Neumayr's assumptions 

 I should consider a certain limitation necessary. As appears 

 from the last of the passages above cited, Neumayr regards 

 the more imperfect and coarser construction of the arenaceous 

 forms in comparison with the isomorphous calcareous ones as 

 a primitive condition, and a special proof that the arenaceous 

 forms are to be regarded as forerunners of the calcareous. In 

 most instances, in all probability, this is the case, but not 

 without exception. It is possible, nay, highly probable, that, 

 as at the present day, the shell-material varies in certain forms 

 with changes of the external conditions under which the 

 Rhizopoda in question live ; this has also occurred now and 

 again during the phylogenetic development, and calcareous 

 forms may thus be compelled to make their shells of sand. 

 These will then, in consequence of the coarser material, 

 appear ruder and less differentiated than the calcareous stem- 

 form. Although the sarcode-body of such forms will have 

 inherited the tendency to secrete hard parts equally well- 

 developed morphologically, it will be unable, on account of 

 the coarser nature of the sandy material, to bring this faculty 

 to full development, as was the case with the calcareous 

 material. Just as the Thalamophora in the course of their 

 phylogenetic development were compelled, for the purpose of 

 the higher morphological development of their shells, to pass, 

 independently in the different stems, from the agglutinated 

 material, which no longer sufficed for this purpose, to carbo- 

 nate of lime, a form which is under the necessity of going 

 back from the calcareous to the arenaceous development will 

 also show a corresponding retrogression in respect of morpho- 

 logy. Such a change of material, as also the existence of 



