Mr W. Macgillivray on the Covering" of Birds. 



Fig. 9. Part of! a pectoral feather of Diomedea exulans, shewing an 

 accessory feather consisting of a small pointed lamina, 

 margined with a few downy barbs. 



10. Sections of three barbs of a quill of Diomedea exulans, a 



little magnified, shewing the mode in which the barbs 

 are connected by the barbules. 



11 . A barb of a primary quill of Tetrao lagopus, viewed laterally. 



12. Part of a barb of a primary quill of Falco fulvus magnified, 



shewing the barbule, connected by barbicels. 



13. The same of the natural size. 



14. Part of a barb of a posterior dorsal (or train) feather of 



Pavo cristatus, magnified, shewing simple barbs. 



15. The same of the natural size. 



On Isopt/re, a new Mineral Species. By W. Haidingee, Esq. 

 F. R. S. E. Communicated by the Author. 



1. Description. — Regular forms not observed. Very pure 

 masses of considerable size, often nearly two inches in every 

 direction, occur imbedded in granite. 



Cleavage none. Fracture conchoidal ; highly perfect, where 

 the mineral is pure ; of lower degrees of perfection, where there 

 are foreign admixtures in it. 



Lustre vitreous, often considerable. Colour greyish-black and 

 velvet black, occasionally dotted with red, as in the heliotrope. 

 Streak pale greenish-grey. 



Opake, or very faintly translucent on the thinnest edges, with 

 a dark liver-brown tint. 



Brittle. Slight action on the magnetic needle. 



Hardness =5.5... 6.0. Specific gravity =2.912. 



2. Observations. — Several specimens of the species of Isopyre 

 are preserved in the cabinet of Mr Allan. Some of them are 

 quite pure, and have no rock attached to them ; others are im- 

 bedded in a kind of granite, chiefly consisting of quartz, crystals 

 of which often penetrate the dark coloured mass of the isopyre. 

 Some of the specimens were procured by Mr Allan three years 

 ago, on a journey through Cornwall, in which I had the plea- 

 sure of accompanying him, from a miner in St Just ; others 



