OF THE LEMURS, BABOONS, AND MAN. 



307 



Angles: Unit prism angle, 101 A 1TO=6030'; prism to brachypinacoid, 110 A 

 010=59 45', macrodome to base (from twin) 101 A 001 =32 30'; angle of twin 67; 

 brachypinacoid to base 010 A 001 =90; prism to base 110 A 001=90. 



368 



369 



FIGS. 367, 368, 369, 370. Lemur catta Oxyhemoglobin. 



Habit tabular on the base (text figures 367 and 368), the crystal consisting of the 

 unit prism and the brachypinacoid, cut by the base. The crystals twin three together 

 on the prism faces, as is common in aragonite, and form almost perfect hexagonal plates 

 (text figure 369). The simple crystals, when the prism and brachypinacoid are in equi- 

 librium, are also almost perfect hexagonal plates. The plates pile up on the base, and 

 evidently produce twins in that way also, in the same orientation as the twins on the 

 prism, simply producing an overgrowth of one crystal on another in the composite group. 

 This produces such an averaging of the elasticities, when the light passes through a 

 number of such lamellae, that the crystal appears to be uniaxial; and, the angles aver- 

 aging also, it becomes, around the edges, where the reaction of one layer upon the next 

 is most pronounced, practically hexagonal (mimetic hexagonal). The crystals grow in 

 rosette-shaped groups (really spherulitic aggregates) and also in single crystals; this 

 formation of rosette-shaped groups is perhaps partly due to the tendency to twin on the 

 macrodome, the second type of twinning. These twins of the second type are the contact 

 hemitrope twins on the macrodome (text figure 370) that have furnished the data for 

 calculating the vertical axis c. The angle of the bases in the two members of this twin 

 is about 67, but was not obtained within less than a degree. The prismatic crystals 

 look like bundles of needles; they were seen crossing each other at definite angles of 

 about 66 to 67 corresponding to the angle of the macrodome and probably represented 

 the base and brachypinacoid, forming a pseudo-prism. Other prismatic groups were 

 seen crossing at a definite angle of 83; this would correspond to twins on the macrodome 

 (403), and the angle of this macrodome on the base would be 41 30'. 



Pleochroism on the base of the tabular crystals is not noticeable. On edge it is 

 more definite, but the pleochroism is weak. Colors are: a rather strong red; & = c, 

 deep red; but they are difficult to observe on account of the deeply colored plasma. The 

 double refraction is weak on the flat, and often, in this position, the crystals are singly 

 refracting, owing to the tendency to mimetic twinning which makes them appear uniaxial. 

 This tendency to mimetic twinning is stronger near the edges of the plate, and in the 

 center the double refraction is generally noticeable. Probably the edge is a true mimetic 

 twin by rearrangement of the crystal molecules, while the center has not been so much 

 affected. In plane polarized light, the extinction is symmetrical on the base and straight 

 in edge views. In convergent light, a simple uniaxial cross is seen in many cases near 

 the edge of the plate; but the biaxial figure usually shows in the central position, with 

 the brushes close together, and 2' = 15 about. The orientation of the elasticity axes 

 is a =6, b=b, c=a. The plane of the optic axes is the brachypinacoid (010), the acute 

 bisectrix is the axis of greatest elasticity, Bx a a, and the optical character is hence 

 negative. 



