376 Prof. Lewis, On the [May 14, 



intersections of known zones. The same reason has led me to 

 doubt the indices assigned to most of the planes to which a note 

 of interrogation is attached. The indices were in these cases 

 generally deduced from measurements of the angles made with 

 prominent planes in the zone of symmetry. 



Crystal 15. Fig. 13. 



A twin-crystal of habit and forms given under crystal 13. The 

 one portion is drawn with the axial system used in Figs. 4 and 12. 

 The portion, whose faces are denoted by the Greek letters equiva- 

 lent to the italics on the first, is obtained by rotating the first axial 

 system about the normal to % (213). The crystal is imbedded in 

 the midst of others, and only so much as is shown in the figure can 

 be seen. Measurement was difficult owing to the want of lustre 

 and the depth of striation on the planes a, d, s, and the unevenness 

 of c and <y. There was a smaller twin of the same kind on the 

 specimen. 



Such approximate measurements as were obtained are compared 

 in the following table with those calculated on the supposition 

 that the individuals have | (213) for twin- and composition-face. 



Calculated 

 cy 76° 23' fairly good 76° 2' 



(76° 56') 



Two small crystals, supposed to come from South America, were 

 recently measured by my friend Mr H. A. Miers of the British 

 Museum. He has kindly communicated to me the results he 

 obtained. The forms observed were acogdstwaik^. The crystals 

 were much striated and distorted, k being the best face, and the 

 faces which give clear definite images were cotwak. 



Since finishing my calculations I have to thank Professor Groth 

 for sending me the specimen in the Strassburg collection, measured 

 by Mr Friedlander, and of which a figure is given in his catalogue. 

 It consists of two very fine crystals attached together. Besides 

 the forms quoted by Mr Friedlander the forms /3 (013) and £(215) 

 are well developed on the crystal. Traces of b, rj, x, and k were also 



