GENERAL CRYSTALLOGRAPHIC CHARACTERS. 331 



plates that are to be seen in the photomicrographs of the rat crystals. 

 In this case the habit of the crystal is tabular on two opposite prism faces 

 of the orthorhombic crystal, and three of these overlapping each other 

 form a hexagonal plate, as may be seen by reference to text figure 210. It 

 only requires the substance of the composite to fill up the reentrant angles 

 to make a perfect hexagonal plate. 



In the crystals of many other groups of animals, similar development 

 of angles of 60 on account of pseudo-hexagonal structure of the substances 

 has been noted, and many examples of this kind of mimetic twinning will 

 be found in the descriptions of the crystallography of the hemoglobin of 

 the species. 



The angle of 90 occurs in many groups as a characteristic angle of 

 the crystals. Thus, in the birds, the Anseres, Gallince (with the exception 

 of the guinea-fowl, which probably does not belong to this family), and 

 Columbce show probable examples of the formation of tetragonal crystals 

 by the mimetic twinning of orthorhombic crystals with angles that approxi- 

 mate 90, and in a few cases the mechanism of this twinning is apparent. 

 The orthorhombic members are the whistling swan with an angle of prism 

 of 88 (92), the chicken with an angle of 87 (93), the quail with an angle 

 of 88 (92), and the pigeon with an angle of 89 10' (90 50'). In the goose 

 and trumpeter swan the only crystals observed were tetragonal crystals 

 with angles of 90, but these were probably mimetic twins of an orthorhombic 

 form. The same kind of twinning that has been described in the rodents as 

 the "horse-type," in which the crystals grow together on the base with a 

 common prism-base edge, seems to be the common form here. Averaging 

 of the angles 88 and 92 or 87 and 93 produces a composite crystal of 90 

 prism angle, and at the same time the elasticities for light are averaged so 

 that a mean uniaxial structure is produced. The crystals twin on the base, 

 and when the twin lamellae are thin, as they finally become, the twin struc- 

 ture finally becomes ultra-microscopic and the symmetry is tetragonal. 

 Examples of this were seen especially in the crystals from the whistling 

 swan and the pigeon. 



Another way in which a higher grade of symmetry is simulated was 

 noticed in several cases, but here in general the measurements of the angles 

 were imperfect and the indications were therefore less certain. This kind 

 of mimesie occurred where the angles of the crystal, orthorhombic for 

 example, approximated those of a higher grade of symmetry, isometric for 

 example. A prism and dome combination, with angles near 71, growing 

 under pressure would have the prism so shortened that the prism and dome 

 come into equilibrium; then the crystal would appear to be an isometric 

 octahedron. The pressure that shortened the prism would be acting upon 

 the axis of greatest elasticity, and when equilibrium of form was reached 

 an equilibrium in the optical elasticities might also be effected and the 

 substance would appear isotropic. In this case no obvious twinning took 

 place, but twinning under pressure may still be the explanation of the phe- 

 nomenon. The crystals of the rats, especially of the Norway rat, showed 

 this characteristic, but unfortunately the angles of the prism and dome were 

 not obtained very accurately for this species. 



