lx 
THE VOYAGE OF H.M.S. CHALLENGEE,. 
triaxon is tetractine tetraxon, we shall substitute for it the equivalent “ calthrops ” 
1, a horse’s hoof ; TpvTrda), to pierce through ; a hoof-piercer). The singular and 
plural forms of this word are the same. The “h” is inserted for euphony. 
2. Triod (rpi-oSo?, 17, a meeting of three roads, Lat. trivium ; here abbreviated on 
the example of “ tripod ”). When one actine is suppressed, the remaining three come 
to lie in one plane, and a triradiate spicule or triod, the commonest form of spicule in 
the Calcispongise, results. In von Lendenfeld’s account of the spicule-nomenclature, 
which is taken from that accepted by our conference in London, this form is assigned 
to a triaxon group without notice of the fact that the triaxon group is that of the three 
rectangular axes. 
Modifications ofi the Calthrops. 
Modifications occurring frequently enough for special designations are not met with, 
occasionally one or more of the actines may dichotomise, but no instance is known in 
which this occurs other than as an isolated variation. 
Modifications ofi the Triod. 
As these rarely occur in the Tetractinellida, but are almost restricted to the 
Calcispongise, we shall not need to consider them here. 
Tetracrepid Desma. 
This sclere (PI. XXIX. figs. 7, 8) is formed by the deposition of sdica upon a 
small calthrops (microcalthrops), it presents therefore four rod-like arms proceeding 
from a centre, these (which as they immediately invest the actines of the crepis may be 
termed “ epactines ”) may branch once or oftener, and usually terminate by subdividing 
into tubercles. In the Lithistida generally the desmas are united by the intergrowth, 
without fusion, of such tubercles, or by the growth of the ends of the cladi of one desma 
over some part of another, such as the sides of the epirabd or end of the epactine ; this 
mode of union we shall term “ zygosis,” and the tubercles or laminae by which it is 
effected will be distinguished as “ syzygial,” the union itself being a “ syzygy.” 
Acrepid Desma. 
In one group of Lithistids (Anomocladidae) the desma does not form upon a crepis, 
at least not a spicular crepis ; it presents a massive centrum, with what appears to be 
a large nucleus, and which may indeed actually be the nucleus of a crepidial scleroblast. 
