862 THE SPIRAL SHELLS [ch. 



place. Be that as it may, it is enough for us to observe that, in 

 such a case a^ ours, when a given fluid, (namely protoplasm) is in 

 surface contact with a solid (viz. a calcareous shell), in presence of 

 another fluid (sea- water), then the angle of contact, or angle by 

 which the common surface (or interface) of the two liquids abuts 

 against the solid wall, tends to be constant: and that being so, the 

 drop will have a certain definite form, depending {inter alia) on the 

 form of the surface with which it is in contact. After a period of 

 rest, during which the surface of our second drop becomes rigid by 

 calcification, a new period of growth will recur and a new drop of 

 protoplasm be accumulated. Circumstances remaining the same, 

 this new drop will meet the solid surface of the shell at the same angle 

 as did the former one; and, the other forces at work on the system 

 remaining the same, the form of the whole drop, or chamber, will 

 be the same as before. 



According to Rhumbler, this "law of the constant angle" is 

 the fundamental principle in the mechanical conformation of the 

 foraminiferal shell, and proV-ides for the symmetry of form as well 

 as of position in each succeeding drop of protoplasm: which form 

 and position, once acquired, become rigid and fixed with the onset 

 of calcification. But Rhumbler's explanation brings with it its own 

 difficulties. It is by no means easy of verification, for on the very 

 complicated curved surfaces of the shell it seems to me extraordinarily 

 difficult to measure, or even to recognise, the actual angle of contact : 

 of which angle of contact, by the way, but little is known, save only 

 in the particular case where one of the three bodies is air, as when 

 a surface of water is exposed to air and in contact with glass. It 

 is easy moreover to see that in many of our Foraminifera the angle 

 of contact, though it may be constant in homologous positions from 

 chamber to chamber, is by no means constant at all points along the 

 boundary of each chamber. In Cristellaria, for instance (Fig. 429), 

 it would seem to be (and Rhumbler asserts that it actually is) 

 about 90° on the outer side and only about 50° on the inner side 

 of each septal partition; in Pulvinulina (Fig. 363), according to 

 Rhumbler, the angles adjacent to the mouth are of 90°, and the 

 opposite angles are of 60°, in each chamber. For these and other 

 similar discrepancies Rhumbler would account by simply invoking 

 the heterogeneity of the protoplasmic drop: that is to say, by 



