68 BRITISH FOSSIL SPONGES. 
Pachastrella primeva (Fig. 3,1). Again, one of the rays may be reduced to a slight 
knob, or may not be developed at all, and the spicule becomes tripodal in form 
(Fig. 3, >), and can then scarcely be distinguished from the three-rayed spicules of 
calcisponges, more particularly when the tripodal elevation is reduced, so that the 
three rays are nearly in a horizontal plane. An important differentiation is shown 
in the so-termed trifid or zone-spicules, in which one of the rays is very much 
elongated and enlarged in proportion to the other three. This elongate ray is 
known as the shaft of the spicule, it is straight or slightly curved, and usually 
Fie. 3.—Various forms of fossil siliceous tetractinellid spicules from Upper Greensand, Chaik, and 
Tertiary strata. (a) Regular tetractinellid or four-rayed spicule, Pachastrella Haldonensis, 
Carter. (%) Microspined spicule in which only three rays are developed. (c) Regular four-rayed 
spicule, moniliform, Pachastrella quadriradiata, Carter. (d) Compound trifid spicule, the shaft 
abbreviated, and the head-rays furcate and extended horizontally. (2) Four-rayed spicule of 
Pachastrella primeva, Zitt., in which two rays are incipiently furcate. (f) Simple trifid or zone- 
spicule of Geodites, sp. (g) Compound trifid or zone-spicule of Geodites planus, in which the head- 
rays are furcate. (i) Simple trifid or zone-spicule of Geodites, sp. (i) Simple trifid or zone- 
spicule, moniliform, Geodites Wrightit. (j) Trifid or zone-spicule of Stelletta inclusa, Hinde, in 
which the head-rays are recurved. (4) Trifid spicule in which the head-rays are sharply recurved 
so as to become anchor-shaped. 
tapers to an acute point (Fig. 3,/). At the head or distal end of the shaft the three 
other rays project at various angles. These head-rays vary very much in different 
spicules. They may be straight or curved, conical, acutely pointed, or club-shaped. 
In many cases, one or more of these rays divide and become fureate (Fig. 3, 7), and 
the canal similarly divides, and an arm extends into each ray. The head-rays diverge 
from the summit of the shaft either in a forward direction (Fig. 3, h), obliquely, hori- 
zontally, or they are more or less recurved (Fig. 3, 7,4). Both rays and shaft are 
