CRYSTALLOGRAPHY OF HEMOGLOBINS OF THE RODENTIA. 237 



sometimes forming a six-rayed star, like text figure 211; more commonly forming X-shaped 

 twins by the interpenetration of two prisms, like text figure 225; and also often showing 

 interpenetration, but with one or two of the rays developed on one side of the prism only 

 (text figure 231). The twin in the zone of the brachypyramid can not form mimetic 

 hexagonal crystals because of the flat dome angle. The axial ratio of this species is 

 related to that of the Mus norvegicus groups of rats by the same 5 : 4 ratio that was true 

 of the black rat. Indeed, except for the difference in solubility and in habit of crystal there 

 is no great difference between crystals of the black and the Alexandrine rats, and these 

 rats look like varieties of the same species. The difference in angle of dome on prism, 

 that makes the slight difference in axial ratio, is within the limit of error of the method of 

 observation, and the axial ratios of the crystals of the two species are probably identical. 

 Pleochroism is rather strong, but, owing to the positions of the crystals presented, 

 it is not possible to distinguish between a and b, the colors of which are apparently nearly 

 alike. The pleochroic colors are a and b pale to deeper yellowish-red; c rather deep red. 

 Double refraction is strong and extinction is straight in all aspects that could be observed. 

 The orientation of fl and b could not be observed, but may be assumed to be the same as 

 in the Norway rat, a=b, b =a; the third axis could be observed and gave c =c. No inter- 

 ference figure could be observed, but from the fact that a = b (nearly) it is probable that the 

 axis of least elasticity c is the acute bisectrix, Bx a = c; and the optical character is positive. 



Reviewing these four species of rats it will be seen that they may be 

 arranged in two groups: (1), the Mus norvegicus group, comprising Mus 

 norvegicus and its albino, Mus norvegicus albus Hatai, in which the crystals are 

 orthorhombic and pseudo-isometric, with an axial ratio of 0.7829 : 1 : 0.7332; 

 and (2), the Mus rattus group, comprising Mus rattus and Mus alexan- 

 drinus, in which the crystals are orthorhombic, but not pseudo-isometric, 

 and the axial ratio is 0.7829 : 1 : 0.5864 or 0.7829 : 1 : 0.5880. As noted above, 

 these ratios for the vertical axis stand in the ratio of 5 : 4; but that they 

 are different axial ratios, and not simply different crystal habits, is shown 

 by the twins in the brachypyramid zones following these ratios in each case, 

 thus indicating that the difference is one of the form of structure of the 

 crystal and not simply a difference of development. The oxyhemoglobin 

 of the Mus norvegicus group is not the same substance as the oxyhemoglobin 

 of the Mus rattus-alexandrinus group. 



CANADIAN PORCUPINE, Erethizon dorsatus. Plates 56 and 57. 



The specimen was received from the Philadelphia Zoological Gardens 

 and was in the form of rather hard clots. The clotted blood was ground up 

 in sand and ether, a little oxalate and water added, and the mixture centrif- 

 ugalized. From the clear liquid thus obtained the slide preparations were 

 made as usual. Crystals began to appear within about half an hour after 

 the slides were covered; they were rather small and thin and showed a 

 tendency to dissolve in the solution. These are described as a-oxyhemo- 

 globin. Inside of 20 hours, they had mostly been dissolved from the slides, 

 and their place was taken by the crystals of /3-oxy hemoglobin; in only a 

 few slides were there any of the a-oxyhemoglobin crystals remaining. The 

 second crop of crystals, /3-oxyhemoglobin, appears to be less soluble and 

 more stable than the a-oxyhemoglobin crystals. 



A second preparation was made the next day, a new lot of slides being 

 mounted from the cleansed blood above described, to which an additional 



