OF THE MARSUPIALIA, EDENTATA, AND SIRENIA. 



177 



twin, and has been observed in the oxyhemoglobin of many species that twin in this 

 way, where it is easily seen that the crystal is composite. If both kinds of twins formed 

 in the same crystal, the averaging of the elasticities might be perfect. But, as has been 

 observed in other species that form hexagonal plates (compare rats, squirrels, etc.), the 

 growth of the composite plate by this form of twinning produces an averaging of the 

 angles, so that prism angles that are nearly 60 (58 30' as in this a-oxyhemoglobin) 

 become exactly 60 in the twin. It might be possible that this crystal was an averaging 

 of right- and left-handed forms, resulting in the more symmetrical mimetic twin. From 

 the forms of twinning assumed, the elasticity axis, c, remains always in the same position 

 in all of the members of the composite crystal, and hence the vertical axis, , becomes the 

 axis of less elasticity, and the composite remains positive. 



If the above view of these crystals is correct the substance of the a-oxyhemoglobin 

 and of the /3-oxyhemoglobin may be the same unless perhaps the /?-oxyhemoglobin is a 

 union of right-handed and left-handed crystals of the a-oxyhemoglobin. 



Reduced Hemoglobin of Didelphis virginiana. 



Monoclinic: Axial ratio a : b : t =1.963 : 1 : I; /3 = 66. 



Forms observed: Prism (110), base (001). 



Angles: Prism angle traces on the base of 110 A 1TO=54; prism edge to base 

 110-lTO A 001=/? = 66; base to plane of symmetry or to side prism edge 001 A 110- 

 110=90. 



(.7 



FIGS. 66, 67. Dutelphit mrginiana Reduced Hemoglobin. FIGS. 68. 69. D. virginiana a-Carbon-monoxide Hemoglobin. 



Habit, rhombic plates with oblique sides, composed of base and prism, the crystals 

 generally very perfect and sharp (text figures 66 and 67). They usually occur singly, 

 but also twin with the normal to the base as the plane of twinning and the twin axis 

 normal to the prism-base edge, the composition face being the basal pinacoid. This 

 type of twin, " horse-type," is seen in a-oxyhemoglobin (text figure 51) and is the common 

 twin on the base in all of these monoclinic reduced hemoglobins and oxyhemoglobins, 

 especially when the prism angle is near 60. These twins are often complex and the 

 polysynthetic arrangement is very common. 



The crystals are readily distinguished from the a-oxyhemoglobin by their color, 

 and by the fact that they occur singly and not in parallel growth, as is so commonly 

 the case in the a-oxyhemoglobin. In the photographs they appear as lighter-colored, 

 more transparent crystals than the oxyhemoglobin crystals. Pleochroism is very strong; 

 a very pale violet, nearly colorless; b deep reddish; c deep claret-color to purple. Ex- 

 tinction is symmetrical or nearly so on the flat basal face; on edge looking along the axis 

 6 it is oblique; the extinction angle is a A a = 13, in the obtuse angle. The orientation 

 of the elasticity axes is as follows: The axial plane is the plane of symmetry; a A a = 13 

 in the obtuse angle, b = 6, c A <! = 11 in the obtuse angle. On the base, the interference 

 figure is readily observed, with the two brushes showing, but in somewhat unsymmetrical 

 arrangement, due to the angle of 13 between a and a; the brushes are widely separated. 

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



12 



