220 CRYSTALLOGRAPHY OF HEMOGLOBINS OF THE RODENTIA. 



Habit very thin tabular on the base, with the very short prism determining the 

 outline of the rhombic plate (text figures 181 and 182). The crystals rarely occur singly, 

 however, but in twins of the usual type on an axis in the base and normal to a prism-base 

 edge, the composition face being the basal pinacoid. These occur on any or all of the 

 prism-base edges, making a group that is composed of many individuals and generally 

 of a roughly hexagonal outline (text figure 183). Hexagonal plates of normal develop- 

 ment occur with these twins, similar to the plates of a-oxyhemoglobin; and they are 

 probably mimetic twins of the /?-oxyhemoglobin, due to the complicated groups above 

 described becoming developed into hexagonal plates. Twins on pyramid faces, as seen 

 in the hexagonal a-oxyhemoglobin, were not observed. 



The color was the usual oxyhemoglobin red, perhaps a little darker than for the 

 corresponding thickness in the a-crystals. Pleochroism is very slight on edge and not 

 noticeable on the basal aspect; a deep red, b = c somewhat deeper red. Double refrac- 

 tion on the base is not noticeable, even with the quartz wedge (b = c) ; on edge the extinc- 

 tion is straight and the relative elasticities may be made out with the quartz wedge. On 

 the base no interference figure of any kind could be detected in convergent light, but it 

 is evident that the vertical axis is the axis of greatest elasticity, and that b=c; hence 

 the acute bisectrix Bx a = a, and the optical character is negative. 



On comparing these two types it is evident that the characters of the /?-oxyhemo- 

 globin are such that it would readily become hexagonal by mimetic twinning, the prism 

 angle being exactly 60, and the double refraction of the ^-modification is such that, 

 but for the form of the crystal, it might be hexagonal. In the mimetic twins, produced 

 by piling up of the rhombic plates to build a hexagonal composite plate, it might readily 

 happen, with the very weak double refraction, that the crystal might become more dense 

 in the direction in which the plates are piled, and hence the vertical axis, or normal to 

 the plates, become the axis of greater density or less elasticity, when the pseudohexagonal 

 crystal would become positive. It is hence entirely probable that the two modifications 

 are really one and the same, the a-oxyhemoglobin being a mimetic twin of the ^-oxyhemo- 

 globin and only pseudohexagonal. 



FOX-SQUIRREL, Sciurus rufiventer neglectus. Plate 46. 



The specimen was purchased from a collector at Orlando, Florida, 

 and was bled in the laboratory, oxalated, ether-laked, centrifugalized, and 

 the slide preparations made as usual. Crystals formed rapidly in the slides, 

 and showed no tendency to dissolve. The blood crystallizes more readily 

 than that of the related gray squirrel. The crystals were shown to be typical 

 oxyhemoglobin by the spectroscope. 



Oxyhemoglobin of Sciurus rufiventer neglectus. 



Hexagonal: Axial ratio not determinate. 



Forms: Unit prism (10TO), base (0001). 



Angles: Prism angle 60, prism to base 90. 



Habit, thin tabular on the base, very symmetrical hexagonal plates consisting of 

 the short prism and the basal pinacoid (text figure 184). The crystals occur singly, or 

 in parallel growths and piled groups on the base, the smaller crystals piled concentrically 

 on a larger crystal. In single crystals the thickness of the plate is one-tenth to one- 

 twentieth of the width, but this is very variable. Many single perfect plates are seen, 

 but this varies in different slides. In some cases the plates elongated on two prism faces 

 or along the diameter of the hexagon parallel to a crystal axis, becoming somewhat 

 orthorhombic looking; most of them are almost perfect hexagons. Twins on a first- 

 order pyramid occur, mostly contact twins (text figure 185). 



The color of the plates is variable with the thickness, but pleochroism is very slight. 

 On the base they are singly refracting, and polarize very faintly on edge; the double 



