Scudder.] 336 [March 27 



these valleys, and that the glaciers abrade, modify, and in a measure 

 reduce the prominent portions left by the streams, and give them the 

 well-known glaciated surfaces. 



[I have not space in the narrow limits of this article to consider the valuable 

 and exceedingly numerous contributions of others to the subject of glacial 

 action.] 



Dr. David Hunt explained a theory to account for the de- 

 crease of prognathism in civilized man. 





Section of Entomology. March 27, 1878. 

 Mr. H. L. Moody in the chair. Fourteen persons present. 



Mr. S. H. Scudder exhibited a number of western Acrid- 

 ians in illustration of one type of dimorphism. 



The different forms had in many instances been described as dis- 

 tinct species, and, indeed, been referred to distinct genera ; the short 

 winged forms having usually been placed in Pezotettix, while the 

 long winged types had been described as Callipteni. This peculiar- 

 ity is found in ten or twelve species, mostly occurring in the Missis- 

 sippi valley ; usually there is a very decided difference in the length 

 of the wings, although in the long-winged forms the tegmina seldom 

 surpass or even reach the tip of the hind femora ; and no interme- 

 diate forms occur. In one instance three varieties occur, which are 

 with little doubt to be referred to one species ; and in this case the 

 variety with the longest wings has tegmina which extend past the 

 hind femora. The dimorphic forms of any one species are found at 

 the same stations, and can not be considered racial. Similar 

 dimorphism has long been known in Orthoptera, and this should 

 doubtless serve as a sufficient reason to group together several of the 

 forms of Xiphidium which had been described as distinct species, 

 and the two northern forms of Gryllotalpa, as well as some so-called 

 species of Gryllus and ISTemobius. 



