134 NEW YORK STATE MUSEUM 



Octahedron. A crystal form bounded by eight equilateral, triangular 



faces; derived from a Greek word meaning eight. 

 Orthoaxis. The axis which, in the nionoclinic" system, is perpendicular to 



the plane of the other two. 

 Orthodomes. Domes or horizontal prisms parallel to the orthoaxis. 

 Orthopinacoid. A pinacoid parallel to the vertical and ortho axes. 

 Orthoprisms, orthopyraniids. Crystal forms the faces of which are more 



parallel to the orthoaxis than the form which determines the axial ratio. 

 Orthorhombic. Presenting the symmetry of the ortho-rhombic system, ref- 

 erable to three uninterchangeable axes which are at right angles to 



one another. Orthorhombic crystals are characterized by binary sym- 

 metry in three directions. 

 Pinacoids. Crystal forms composed of planes parallel to two axes and 



corresponding in position to the faces of a cube; the (term is derived 



from a Greek word meaning a board. 

 Plane. One of the bounding plane surfaces of a crystal; the term is 



extended to include the imaginary extension of this bounding surface to 



meet the axes. 

 Prisms. Crystal forms the planes of which intersect two basal axes and 



are parallel to a third. Domes as described above may be regarded as 



horizontal prisms. 

 Pseudohexagonal. Apparently hexagonal; many crystals seem, by reason 



of twinning, to be hexagonal though belonging to a system of lower 



symmetry. 

 Pseudoisometric. Apparently isometric; note above. 



Pseudomoa'ph. A substance having the form of one mineral and the com- 

 position of another; the term is derived from two Greek words meaning 



a false form. 

 Pyramids. Crystal forms the planes of which intersect all three axes. In 



the isometric system forms of this type are designated by special terms 



such as octahedron, trioctahedron, etc. 

 Pyritohedrons. Isometric forms so named from their common occurrence 



in the species pyrite. 

 Radiated structure. Consisting of crystalline individuals which radiate 



from a center. 

 Eeniform. Kidney-shaped; from the Latin reties, a kidney. 

 Reticulated. Interlaced like a net; from the Latin reticulum, a net. 

 Rhombohedrons. Hexagonal forms of trigonal symmetry bounded by six 



rhombic faces. 

 Scalenohedrons. Crystal forms of the tetragonal and hexagonal systems 



bounded by scalene triangles and presenting in general a somewhat 



wedgelike shape. 

 Second order. A term applied, in the tetragonal and hexagonal systems, 



to pyramids and prisms the faces of which are related to those of the 



corresponding first order forms as shown on p.26, fig.98. 

 Sphenoids. Crystal forms of the tetragonal and orthorhombic systems 



bounded by four triangular faces and closely related by analogy to the 



isometric tetrahedron. 

 Stalactitic structure. Consisting of pendant columns or forms resembling 



icicles. 



