Parietes ribbed, mottled purplish-red; orifice broad, trigonal, hardly toothed.
The leaves are trigonal; they are very short, and they are laid over each other like scales in four rows.
When the whorls are angulated, a trigonal aperture is the result, as in many species of Trochus.
Fig. 73 represents a crystal of tourmaline with the trigonal prism , hexagonal prism , and a trigonal pyramid at each end.
Mactracea (plates, fig. 77 to 88), the cartilage placed in a trigonal pit, with a small external ligament.
All other forms, in which the faces are neither parallel nor perpendicular to the triad axis, are trigonal pyramids.
All the prisms are trigonal prisms; and perpendicular to these are two pedions.
The horns are trigonal, laterally compressed and knotted on the upper edge.
trigonal trapezohedron (fig. 74), bounded by six trapezoidal faces.
The cartilage placed in a trigonal pit with a small external ligament.
Relating to a crystal having three axes of equal length intersecting at oblique angles. This crystal system is considered a subset of the hexagonal system. The mineral quartz has trigonal crystals. Also called rhombohedral. See illustration at crystal.