500 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.59. 



rich in forms than the typical or type 1 habit. The dominant forms 

 are the positive orthodome x (102) and the negative hemipyramid 

 e (Tl2). There is a gradation in the opposite direction toward a type 

 more or less prismatic by elongation on the d axis, as shown in 

 figure 4. Crystals as extreme in development as that here illustrated 

 are relatively uncommon. The basal pinacoid, which on the type 1 

 crystals is represented by a small, nearly triangular face, is here an 

 elongate rectangle. Crystals of this habit seldom show a very large 

 development of the base. Certain other crystals which show a promi- 

 nent development of the front pinacoid may be referred to type 3 of 

 Kraus and Cook as illustrated in figure 6. 



Fngemach's definitions are considered to possess an unjustified 

 rigidity. Crystals of his type 2 are abundant in those studied by 

 the writer. They all show a more or less prominent basal pinacoid, 

 as does Kraus's type 4, and they will be referred to as type 4, although 

 the crystals of this habit examined by Kraus and Cook chanced to be 

 the most simple development of the type. The crystals of this type 

 show some variation in habit, the large development of the basal 

 pinacoid being the only constant criterion for their classification. 

 As will be observed from a study of the table of combinations given, 

 the forms regarded by Ungemach as characteristic and diagnostic of 

 his two types frequently coexist on the same crystal. Some of the 

 figures illustrate this, and some short descriptions will serve to point 

 out examples of this, as shown below: 



Crystal 65, illustrated in figure 22: A peculiarly distorted crystal 

 elongated parallel to o (120) has the combination characteristic of 

 type 1, having the forms v (Til), b (223), u (104), and v (103), all 

 diagnostic of type 1, according to the definition, together with the 

 steep positive dome <p (101), as a well-defined though wholly dull face. 



Crystal 39, illustrated in figure 15, is one of the characteristic crys- 

 tals of what is here adopted as type 4. It shows q (312) and s (302) 

 as small dull faces and 2 (302) as a well-defined face. These three 

 type 2 forms are accompanied by the type 1 form v (Til), which is 

 present as brilliant faces. 



Crystal J+3, showing development typical of tj T pe 1 and the diag- 

 nostic type 1 form v (Til), has the type 2 forms q (312) and $ (530) 

 as well-developed faces. 



Crystal 44> a P&rt of which is shown in figure 20, is also a large 

 crystal typically developed like type 1 and showing the type 1 form 

 v (Til), together with the type 2 form q (312) and thenewformu (211). 



Crystal 7, a somewhat broken crystal, has x (102) as the only 

 prominent form in the orthodome zone. This crystal shows one face 

 of q (312) and two small faces of t) (811) with one narrow but distinct 

 face of v (Til). 



