. Carnegie Institution of Washington publication. . 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 or
. Carnegie Institution of Washington publication. . 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'.
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