OF THE CATS AND CIVETS. 289 



The type (a) crystals did not appear to twin, and in the type (c) crystals twinning 

 was only doubtfully observed; but in the type (b) crystals they formed sixlings, appar- 

 ently by twinning on a pyramid (233), the six being arranged with the pseudo-tetragonal 

 pyramid points all pointing outward or in the direction of the axes (text figure 332). 

 The group of six thus formed continued to grow, and the somewhat sunken faces filled 

 up; at the same time, the polar edges of the group became straight lines, so that, finally, 

 the crystal became apparently isometric in form. The group also lost all trace of double 

 refraction and became isotropic, passing into the mimetic form of /3-oxyhemoglobin, 

 which may be an isomer of the a-oxyhemoglobin. 



Pleochroism is not very strong in any of these three types of crystals; but they 

 are all small, so that the colors are necessarily pale. The color varies inversely as the 

 elasticity, as usual; a nearly colorless, b deeper red, but rather pale, c rather deep red. 

 In the type (c) crystals the change of color is very slight. The mimetic crystals show 

 no pleochroism at all. Extinction is straight in type (a) and type (b), but the tabular 

 crystals of the third type are too thin to polarize. On the type (a) crystals, looking 

 along the brachy-axis, the biaxial interference figure is seen, with the separation of the 

 brushes very slight. The orientation of the elasticity axes is a = i, 6=6, c=a. The 

 plane of the optic axes is the brachypinacoid; the axis of least elasticity is the acute 

 bisectrix, Bx a =(, and the optical character is positive. The crystals are nearly uniaxial, 

 the separation of the axes is quite small, 2E = 15 about. 



fi-Oxyhemoglobin of Felis concolor. 



Pseudo-isometric: A mimetic twin of the orthorhombic 

 a-oxyhemoglobin. 



Forms observed: The only form is the apparent octa- 

 hedron. 



Angles: As well as the angle could be measured it was 

 about 71. 



Habit octahedral; the apparent octahedra being, in some 

 cases, very symmetrically developed and the faces very smooth 

 (text figure 333). Their development from the pyramidal- 



... , , , ,, , , , . , i j ., , Flo. 333. Felis concolor 0-Oxy- 



lookmg crystals of the a-oxyhemoglobm has been described hemoglobin, 



under that substance. 



The crystals are a very bright scarlet red, as is usually the case in oxyhemoglobin 

 crystals that show little or no pleochroism. In this case, there is no trace of any pleo- 

 chroism; nor is there any double refraction. The crystals have absolutely no effect upon 

 polarized light; evidently the twinning has produced a structure in which the elements 

 of the twin are subrnicroscopic, or it is a true mimetic twin. 



Reduced Hemoglobin of Felis concolor. 



Orthorhombic: Axial ratio not determined. 



Forms observed: Unit prism (110), macrodome (101), brachypinacoid (010). 



Angles: The prism is nearly square; but neither the prism nor dome angles could 

 be measured in the few crystals seen, because of the positions in which they were lying. 



Habit prismatic on the vertical axis, the combination consisting of the unit prism 

 and the macrodome; very frequently one of the dome faces is much more developed 

 than the other, and this produces a monoclinic aspect. Some of the crystals are tabular 

 on the brachypinacoid, but most of them are flattened on two planes of the macrodome. 

 No twins were observed. 



Pleochroism is very marked ; a nearly colorless, 6 deep rose, c deep red. The extinc- 

 tion is straight with the direction of the crystal axes, in all aspects. On the oblique end 

 section of one crystal, a single brush of the interference figure was seen, and it gave the 

 orientation of the axes as follows: a = a, 6 = 6, c = <!. The plane of the optic axes is the 

 brachypinacoid, the acute bisectrix Bx a = c. The optical character is hence positive. 



19 



