718 Mr. Gilbert J. Arrow on 



In the island of St. Helena is found a peculiar genus of 

 Dynastidas named Mdlissius. This consists of two species, 

 of which one, Mcllissms adumhrUus, has the propygidium 

 produced backwards and covered with very fine and well- 

 developed transverse ridges, Avhile in the other, Mdlissius 

 eudoxus, the same part is reduced in size and its surface 

 is relieved only by coarse scattered elevations which by 

 their transversely elongated form show their derivation 

 from the very different microscopic ridges still perfectly 

 preserved in the other form. This degenerate species is 

 much more common and widespread than the other, and 

 we may suppose tliat, within the restricted area of the 

 island, stridulation, owing to some unknown conditions 

 prevailing there, is rendered useless or disadvantageous, 

 and that, a race in which the stridulatory apparatus is 

 atrophied having arisen, the older form is gradually 

 disappearing before it. It was observed by their first 

 describe!', Wollaston, that if. eudoxus produced no audible 

 sound, but he seems to have had no opportunity of hand- 

 ling living specimens of the rarer form. 



Other genera possessing an apparatus of the Oryctes 

 typesiYe Xylorydcs, Sccqmnes, Stypotrupcs, Ci/pJionistes, Didio- 

 dontus, Hderogouiphus, Podisdmus, Tlironistes, Corynoscdis, 

 Augosoma, Londiotns and Dipelicus. 



In the large genus Hdcronydius different species show 

 an interesting transition from simple forms in which 

 punctures originally existing upon the propygidium have 

 become drawn out laterally into furrows, covering a large 

 part of the surface and leaving fine but not very reguLir 

 ridges between them, to more numerous species in which 

 the ridges are perfectly regular, much more delicate and 

 restricted to a narrow longitudinal band on each side, the 

 two bands always converging and becoming more coarsely 

 ribbed as they approach the anterior border of the seg- 

 ment. From this elaborated type the structure may be 

 traced through progressively coarser forms until at last 

 we find the component ridges large and separated by 

 intervals several times their own diameter, so that any 

 sound produced fiom them must be more a rattle than a 

 squeak. In species of the allied genus Pcntodon the 

 ridges become still more scattered and irregular, and a 

 similar degenerate condition seems characteristic of Ccdo- 

 sis. Oidy practical observations can prove whether these 

 scattered ridges are merely the vestiges of an organ which 



