CRYSTALLOGRAPHY OF HEMOGLOBINS OF THE RODENTIA. 



231 



210) and there may be more than three in the combination. This kind of grouping pro- 

 duces roughly hexagonal plates, and even fairly regular hexagonal tabular crystals, in 

 which the composite character can, however, usually be made out. Rarely, this com- 

 posite character almost disappears when the members of the twin are many and very 

 thin, and the crystal then becomes pseudohexagonal; this is the normal hexagonal 

 crystal of the rodents. When, on the other hand, the crystals twin on edge, they seem 

 to be more interpenetrant; although in these, too, there is often the appearance of piling 

 up. The twins of crystals on edge are in the form of six-rayed star-shaped groups (text 

 figure 211). In some cases the two aspects of the crystals are presented in the same 

 group, as is natural, for there is no essential difference of structure on the two kinds of 

 prism faces, the broad face and the narrow face. The twin on the flat seems to be on a 

 brachypyramid nearly, or quite in the zone of the prism-dome edge, but the composition 

 face is the unit prism; in the star-shaped twins, the twin plane and composition face 

 are a pyramid of the unit series. 



207 



208 



Fios. 207, 208, 209. 210, 211. Mia nonegicvi aibus Oxyhemoglobin. 



The crystals therefore occur in five habits or forms: 



(1) Prismatic crystals, long or short, consisting of the symmetrically developed 



prism and generally unsymmetrically developed brachydome. Rather rare, 

 but the most nearly symmetrical crystal. 



(2) Elongated six-sided plates formed from (1) by flattening on two opposite 



prism faces. The common single crystal. 



(3) Composite and rough hexagonal plates, twins of (2) on the brachypyramid, 



presenting the tabular aspect. The common crystal. 



(4) Six-pointed star-shaped twins, produced by twinning on the unit pyramid, 



presenting the narrow planes or edge aspect. Almost as common as (3). 



(5) More or less regular hexagonal plates, mimetic twins of the type of (3). Rather 



rarely observed. 



The color of the crystals is rather pale, owing to their very small size, but pleo- 

 chroism is quite noticeable; a and 6 nearly equal, almost colorless in these minute crys- 

 tals; c is reddish-orange to deeper red. The orientation of the elasticity axes is only 

 approximately made out for a and 6, which are nearly equal, but t = 6. The peculiar 

 development of the crystals renders it nearly impossible to get views along axes a and b; 

 there can be little doubt, however, that the acute bisectrix Bx a c, which would make 

 the optical character positive. 



