92 PROCEEDINGS OF THE ACADEMY OF 



behind, by smaller scuta, three in front, two at the side, and two 

 behind. Commencing with tiie first. No. 1 has already been de- 

 scribed. No. 2 is small, oval, and antero-posterior ; No. '> is an 

 antero-posterior pentagon, with the narrowest side inwards. No. 

 4 is a similar transverse pentjtgon. No. 5 is an antero-posterior 

 pentagon, which presents its shorter lateral facet inwards. No. 6 

 has a similar character, but is smaller and with more definite an- 

 gles. Another series of scuta is seen outside of these at one end 

 of the series. Three of this set bound the front and side of each 

 of the median pair above mentioned, leaving a short facet next 

 its fellow unaccounted foi*. The sculpture consists of radiating 

 ridges and' tubercles, which are most broken near the centres of 

 the scuta. The tubercles and ridges are obtuse and low, and the 

 latter do not inosculate. An angular depression commenced at 

 the middle of each lateral area, and extends across the middle line 

 at the point of junction of the paired and single median scuta. 



M. 

 Width of vertex at middle scuta . . . .0.176 

 Length of single median scute .... .048 



June 30. 

 The President, Dr. Ruschenberger, in the chair. 

 Fifteen members present. 



Permission having been granted (the meeting being for busi- 

 ness), Prof. Persifor Frazer, Jr., made the following remarks : 



In the investigation of the chemical formulas of minerals, the 

 student will meet with two kinds of difficulties. The first is the 

 great variations in the analysis, and the next is the connecting to- 

 gether in the formula for the particular mineral of different com- 

 pounds by the sign -|-. The first of these difficulties is a neces- 

 sary consequence of the manner of formation and occurrence of 

 minerals in tthe midst of solutions of other materials, and conse- 

 quently subjected to mechanical and chemical conditions tending 

 to add impurities to it, (whether by percolations tlirough its open 

 joints, crevices, and pores, or by oxidizing or partially decompos- 

 ing it). 



The first results of the best processes of mechanical separation 

 merely reduce to. a low per cent, the admixture of one mineral 

 with another of greatly difterent sp. gr., and even the best known 

 chemical methods can never succeed in producing an absolutely 

 and theoretically pure substance. So much less probable is it that 

 the deposits of compounds by the mechanical and chemical pro- 

 cesses which a change of the conditions of the surrounding nature 

 have caused, and which have not been bottled up in impervious 

 glass jars, but left to the action of the rain and sun and subter- 



J 



