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