OTARIID^E, PHOCID.E, MUSTELID^E, PROCYONID^J, AND URSID^J. 255 



the crystal has then a normal monoclinic habit (text figure 271) . In this form they appear 

 to be quite permanent, and do not disappear from the slides for more than 2 weeks. 

 These equidimensional crystals are largely of a second growth, however, and the tabular 

 type may persist in that form until they attain considerable size. When still in the 

 tabular form, and within a day or two after the preparations are made, these crystals 

 very frequently twin on the hemiorthodome as contact twins (text figure 272), showing 

 often, too, groups of three in contact, perhaps twinned on both hemiorthodomes. As 

 the crystals develop larger unit-prism faces they also form contact twins on the prism. 

 Penetration twins of these short prismatic crystals, showing only the prism and base, 

 are seen (text figure 273) twinned on the same positive hemiorthodome. Some of the 

 plates or tabular crystals of type (a) that appeared among the first crystals, show the 

 prism (430) instead of the unit prism; these do not seem to be common, and were not 

 observed in the later crystals of this type that formed in the slides. 



Crystals of type (b) : These appeared, within 3 days after the preparations were 

 made, in the slides that had been kept in the cold. The habit of these type (b) crystals 

 is long to stout prismatic, elongated on the vertical axis (text figure 274) ; and they 

 attain a length of 20 times that of the largest of the type (a) crystals. Their ratio of 

 length to thickness varies from 25 : 1 to 4 : 1. They are evidently the unit prism termi- 

 nated by the base and the hemiorthodome; but most of them are contact twins on the 

 prism face (text figure 275), and the angle of the prism edge to base is likely to show on 

 both sides of the termination. In some of these crystals the clinopinacoid appears to be 

 present. Besides twinning on the prism they also appear to form penetration twins on 

 the pyramid. These crystals are more soluble than those of type (a) ; they were formed 

 in the cold and began to dissolve when brought into a warm room. They were the first 

 crystals to disappear from the slides, and may contain more water of crystallization, or 

 water of crystallization instead of serum of crystallization. They were oxyhemoglobin, like 

 the type (a) crystals, but the spectrum was not measured for possible displacement of the 

 lines, which might be expected to occur if the liquids of crystallization were not identical. 

 The crystallographic characters appeared to be the same in both, which is to be expected 

 with the same molecular structure, no matter what the crystal liquids may have been. 



Pleochroism is marked in both types, which exhibit the same optical characters 

 throughout, except that the prismatic type (b) usually presents one orientation with 

 the prism face normal to the line of sight. The pleochroic colors are: a nearly colorless, 

 b and c nearly equal and deep red. Extinction is symmetrical on the base, or when 

 observing the crystal in the direction of the plane of symmetry; when observing normal 

 to the plane of symmetry, the extinction angle is 15 measured from the trace of the 

 base or 20 28' from the prism edge by calculation, usually measured as about 20. On 

 the crystals observed normal to a prism face, the extinction is about 12 as usually 

 observed, measured from the prism edge; in the type (b) crystals twinned on the prism 

 this becomes 12 in each half of the twin, symmetrical with the composition plane. In 

 convergent polarized light a brush of a biaxial interference figure was seen in some posi- 

 tions, especially when looking at the obtuse angle between the base and the front prism 

 edge. The orientation of the elasticity axes is a A ^ = 20 28', in the obtuse angle; b=6; 

 c A a = 15, in the obtuse angle. The plane of the optic axes is the plane of symmetry 

 and the axis of greatest elasticity is the acute bisectrix, Bx a =a; the optical character is 

 hence negative. The axial angle is evidently large, but a single brush is seen on the cross- 

 section of the prism and also on the basal section, but both are excentric. 



OTTER, Lutra canadensis. Plate 66. 



The specimen of blood was received from the National Zoological 

 Park of Washington, District of Columbia. The blood was oxalated and 

 repeatedly frozen and thawed, and from this solution the slide preparations 

 were made. The blood crystallized rather readily, and the crystals did not 

 dissolve upon slight increase of temperature. They were oxyhemoglobin. 



