298 C. E. Beecher — Pygidium of Triarthrus. 



banded envelopes of a yellow mineral between which and the 

 green is a powdery white coating, the whole forming a large 

 nodular mass nearly seven inches in its longest diameter. A 

 partial analysis of the yellow mineral and the white powder 

 which separates it from the green, gave for the yellow, 



H o 16-89 



P„"0 6 34-43 



and for the white powder 



H 2 19-74 



P 2 5 37-43 



showing a progression from yellow through white to the green. 

 Both the white and yellow substances leave an insoluble residue 

 which is larger in the yellow, and as the relation between the 

 water and acid is about the same in all three cases it seemed 

 probable that the three are practically the same mineral, the 

 green being pure, the other two carrying an impurity. A thin 

 section of the green mineral viewed by polarized light shows 

 it to have a finely granular structure with an occasional spher- 

 ulitic area. The grains do not present well defined crystallo- 

 graphic features but are irregular in outline, and of different 

 sizes. 



The banded mineral also shows a granular structure but the 

 section is traversed by streaks of an opaque yellowish sub- 

 stance to which the prevailing yellow color of the bands is 

 doubtless due. The specific gravity of the green mineral is 

 2*62 and its hardness about 4. 



No information was furnished which would throw light on 

 the question of the source or genesis of this mineral, of which 

 the present appears to be the second publicly known occurrence 

 in this country. 



Washington, D. C, Jan. 25, 1894. 



Aet. XXYIIT. — The Appendages of the Pygidium of Triar- 

 thrus ; by Chaeles E. Beecher. (With Plate VII.) 



Since the previous publications in this Journal, on the 

 antennae and thoracic legs of Triarthrus Bechi* additional 

 features have been worked out from the material in the Yale 

 Museum, and it is now proposed to give the main characters 

 of the appendages attached to the pygidium, or caudal shield. 

 The structure of the under side of the head will next be de- 



* Antennae and other Appendages of Triarthrus Beckii; by W. D. Matthew. 

 This Journal, August, 1893. On the Thoracic Legs of Triarthrus; by C. E. 

 Beecher. This Journal, December, 1893. 



