ortho tonus (ortho tonus)

[NL., straight, + τρίαινα, a trident.] In the nomenclature of sponge-spicules, a triæne whose three clad i or prongs project at right angles with the shaft; a simple spicule of the rhabdus type, trifurcate or with three secondary rays at one end, and these rays at right angles with the shaft. Sollas.