XX. Proceedings. [March 21st, 1^22, 



and Bow Type. The fiddle portion generally takes the form 

 of a row or series of rows of regularly arranged granules or 

 tubercles or a file-like series of ridges. The bow portion is 

 either a sharp smooth ridge or a row of granules. The rapid 

 motion of the fiddle across the bow or vice-versa produces the 

 sound. (3) Plectrum Type. The sound is produced by the 

 friction or rubbing together of two series of stiff, modified, 

 thickened and hollow spines. The first type is found in the 

 snapping shrimps (Alpheus) so characteristic of the coral reefs 

 of tropical countries. Examples of the second type are met 

 with in the spiny lobster of our own coasts, several species of 

 shore crabs from tropical waters like Matuta, Platyonichus, 

 Pseudozius and the interesting amphibious crabs, Ocypoda and 

 Uca, the denizens of tropical mud flats. The third type of 

 sound-producing mechanism is found only in certain river crabs 

 in Africa. 



In all cases these stridulating organs are found in both sexes 

 of a species. It is difficult to say what their function is, but in 

 those instances where they have been most carefully studied — 

 the crabs of the genus Ocypoda — there is evidence that the 

 sound is produced as a warning-note to keep would-be intruders 

 from entering a burrow already occupied. 



Ordinary Meeting, March 21st, 1922. 

 Mr. T. A. Coward, M.Sc, F.Z.S., F.E.S. {President), in the 



Chair. 

 A critical account of 



" Wegener's Theory of the Origin of Continents and 

 Oceans " 



was presented by Mr. W. B. Wright, F.G.S., and Professor F. 

 E. Weiss, D.Sc, F.R.S. 



Mr. W. B. Wright gave an introductory address outlining 

 the main features of Wegener's theory. This is merely a 

 development of the modern theory of isostasy, which presumes 

 an adequate degree of plasticity in the surface layers of the earth 

 and the existence of a state of gravitational balance between its 

 various parts. The lighter continental masses are assumed to 

 be afloat on the heavier viscous basaltic substratum, and to be 

 capable of lateral movement over it. The geological reconstruc- 

 tions thus rendered possible can be made to explain many 

 geological and biological anomalies, which former theories have 

 only dealt with in a clumsy and disconnected manner. A fairly 

 successful attempt is made to show that if the reconstruction of 



