OF PISCES, BATRACHIA, AND REPTILIA. 



155 



The pleochroism is very noticeable in the twinned plates where the twinning is the con- 

 tact twin of the "horse-type." The part where one crystal overlaps the other may be 

 of a brownish-yellow color when the two parts of the single crystals projecting from the 

 common part are respectively deep red (c) and pale lilac (near a). All parts show the 

 reduced-hemoglobin spectrum, however, the absorption band varying in width and the 

 limit of absorption varying with the different colors, as seen in the different parts of 

 the composite crystal. The axial plane lies in the plane of symmetry, and the orienta- 

 tion of the optic axes is similar to that of the oxyhemoglobin, etc. The mean elasticity 

 axis 6=6, a A a = 14; c A 6=Q, both in the obtuse angle. Extinction is straight on 

 the base, and on edge view, looking along 6, the extinction is 14 from the trace of (001) 

 and 6 from the edge of (1 10) . From these maxima the extinction varies down to parallel 

 in other edge views; this is readily observed in twins. In the star-shaped twin the 

 opposite sectors extinguish simultaneously. Upon the basal aspect the interference 

 figure is seen with one brush in the field and the other outside of it. The axial angle 

 is large as in the former cases. Optical character positive, Bx a =c. 



Reduced hemoglobin crystals developed in all bloods that had been kept for some 

 days, either at ordinary temperature or in a refrigerator. They usually developed in blood 

 that had contained oxyhemoglobin, or had been exposed to the air before the reduction 

 to hemoglobin began. In short, the two forms above described seem to pass into reduced 

 hemoglobin when in solution and even when in the form of crystals, the latter by para- 

 morphic change. But evidently most of the crystals of reduced hemoglobin examined 

 crystallized from solution as such. 



These reduced-hemoglobin crystals were often associated with the pure methemo- 

 globin crystals, in regular growths, in the same way that was noted in the case of oxy- 

 hemoglobin, text figures 14 and 15. 



(4) Methemoglobin of Alosa sapidissima. 



Hexagonal: Axial ratio not determined. 



Forms observed: Prism (10TO), base (0001). 



Angles: Prism 120 (60); prism to base 90. 



Habit tabular, in hexagonal plates, by strong development of (0001) (text figure 

 13). The plates grew at first singly, but afterwards developed upon the crystals of oxy- 

 hemoglobin and of reduced hemoglobin, when these latter by twinning and adjustment 

 had developed angles of 60 

 and 120, and these regular f 



growths are symmetrical, the 

 methernoglobin growing on 

 the sides and angles of the 

 monoclinic crystals, with the 

 basal surfaces parallel in the 

 two substances (text figures 

 14 and 15). This brings the 

 axes of least elasticity almost 

 parallel in the two. The met- 

 hemoglobin often completely 

 incloses the monoclinic crystal, which can be seen through the enveloping layer, owing 

 to its strong pleochroism and double refraction. 



Pleochroism is rather weak, seen only on edge view; the colors are e deep brownish- 

 red, aj paler, but differing only in shade. In convergent light the uniaxial figure is seen 

 on the basal aspect; in parallel light the crystal is singly refracting in this aspect. Ex- 

 amined on edge, e has less elasticity than w. Hence s>ta and the crystal is positive. 



From the characters of these methemoglobin crystals, it is very likely that the sub- 

 stance is really a mimetic twin and only pseudohexagonal. It is not very permanent, 

 but decomposes and produces a granular brownish precipitate, leaving the monoclinic 

 crystals (usually now changed to reduced hemoglobin) unaltered. 



M 



FIGS. 13, 14, 15. Alosa iapidiesima Oxyhemoglobin. 



