OF THE MARSUPIALIA, EDENTATA, AND SIRENIA. 



175 



gas and the oxygen displaced by CO as shown by the spectroscope, after 

 which it was centrifugalized. Most of the preparations examined were 

 made in the usual manner of laking the oxalated blood with a few drops 

 of ether and centrifugalizing for several hours. From the fresh blood the 

 first crystals to form were always a-oxyhemoglobin, but from stale blood 

 reduced hemoglobin formed. The regenerated stale blood gave only a-oxy- 

 hemoglobin at first, but later the crystals of the other hemoglobins appeared. 



a-Oxyhemoglobin of Didelphis virginiana. 



Monoclinic: Axial ratio a : b : 6 =1.7856 : 1 : 2.6685; /3=48 (about). 



Forms observed: Prism (110), base (001), orthopinacoid (100), pyramid (111), 

 orthodome (T01). 



Angles: Prism-base edges 110-001 A lTO-001 =58 30'; 001 A 010=90; 001 A 

 100 =/3 =48, or perhaps somewhat less; T01 A 001 =about 90. The angle of the ortho- 

 dome to the base was not exactly determined. The angle ft was only determined approxi- 

 mately, and some of the crystals measured showed the angle to be apparently about 43. 



FIGS. 58, 59, CO, 61, 62, 63, 64. Didelphit virginiana a-Oxy hemoglobin. FIG. 65. Didelphis virginiana 0-Oxyhemoglobin. 



Habit tabular parallel to the base (text figures 58 and 59), the first crystals to form 

 being very thin and very symmetrical rhombic tables; as they increase in size they develop 

 other planes besides the simple prism and base, which is the usual crystal. The first 

 plane to appear, besides these two, is generally the positive hemiorthodome (T01) which 

 cuts off the ends, or sometimes mainly one end, of the rhombic plate (see plate 16, fig. 91) . 

 The best crystals were obtained from the laked corpuscles and these showed in some cases 

 the negative hemipyramid (111), in addition to the orthodome, beveling the edges of the 

 plate in the obtuse angle (text figure 60) . Composite groupings formed by parallel growth 

 are common, especially in the direction of b. Twins are of several kinds, but are not so 

 frequently observed as is usually the case in these rhombic plates of the monoclinic sys- 

 tem. A common form is a twin with the twinning axis parallel to the prism-base edge 

 110-001, and the base as the composition face (text figure 61). These form the symmet- 

 rical twin shown on plate 16, fig. 92. The similar twin on the base as composition face 

 and the twinning axis normal to prism-base edge and lying in the base, and the plane of 

 twinning normal to the base, was also observed (text figure 63). This twin with the 

 composition face the base is the usual type of twin in such monoclinic rhombic plates 

 (text figure 62) (horse-type; see description of this twin under Horse). A third type is 



