926 General Notes. [November, 



The Archipolypoda of the Mazon creek nodules have stout forked 

 spines and two pairs of legs to each segment. Palaeocampa may 

 therefore be considered as a precursor of the Chilopoda, just as the 

 Archipolypoda are the precursors of the Diplopoda, and the 

 discovery of these types prove that at this early period the di- 

 vergencies of structure among myriapods were as great as they 



are to-day. In the same journal Messrs. McGee and Call write 



upon the Loess of Des Moines, Iowa, giving faunal tables of the 

 fossils, which have a less aquatic facies than the modern mollusca 



of the same district. Messrs. Scott and Osborn describe Ortho- 



eynodon, an ancestor of the Rhinoceros, from the Bridger Beds 

 of Wyoming. It is the oldest known representative of the line, 

 and differs from Amynodon in the erect lower canines, similarity 

 of premolars and molars and other particulars. He has little of 

 the rhinocerotic character in the skull, but the possession of ca- 

 nines and loss of the median incisors point it out as related to 

 Amynodon. 



MINERALOGY. 1 



The action of Heat upon Crystals ofBoracite. — Mallard con- 

 tributes to the Mineralogical Society of France an interesting paper 

 upon the change which heat produces in the optical properties of 

 boracite. The leading mineralogists of Germany — Klein, Zirkel, 

 Groth, etc., hold that such changes are due simply to unequal in- 

 ternal tension, such as may be produced in glass. That the bi- 

 axial character caused by heat is an essential and characteristic 

 character of boracite, due to the twinning of twelve rhombic 

 crystals around a point, is again strongly urged by Mallard in the 

 present paper. He supposes each individual of the pseudo-sym- 

 metrical crystal to be a pyramid whose base forms a face of the 

 external dodecahedron, and whose summit is at the center of the 

 crystal. By cutting sections in various directions through the 

 crystal, and examining them optically both before and after heat- 

 ing, he shows that there are persistent optical properties which 

 cannot be explained by irregular tension. He concludes that by 

 the action of an intense and prolonged heat, a series of very thin 

 plates are formed, alternately twinned with each other according 

 to a definite crystallographic law. He shows that analogous phe- 

 nomena may be produced by the action of heat upon sulphate 

 of potash, and that such invariable phenomena could not be pro- 

 duced by tension or pressure in a colloid substance. 



Prehnjte.— This zeolite, so frequent in rocks of igneous 

 origin, and recently found to so frequently exhibit curious opti- 

 cal properties, has been carefully described by Professor B. K. 

 Emerson in its associations and alterations in the Deerfield Dike 

 of Connecticut. Prehnite is regarded as the oldest mineral in the 

 veins in which it appears. Frequently the motion of the rock 

 walls produces slickensides upon the prehnite, and sometimes 



1 Edited by Professor H. Carvill Lkwis, Academy or Xatural Sciences, Phila- 



