68 



BRITISH FOSSIL SPONGES. 



Pachastrella primatva (Fig. 3, I). 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, b), 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 



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8*^1 



/ 

 x4o 



Fig. 3. — Various forms of fossil siliceous tetraetinellid spicules from Upper Greensand, Chalk, and 

 Tertiary strata. (a) Regular tetraetinellid or four-rayed spicule, Pachastrella Haldonensis, 

 Carter. (£) Microspined spicule in which only three rays are developed, (c) Eegular four-rayed 

 spicule, moniliform, Pachastrella quadriradiata, Carter, (d) Compound trifid spicule, the shaft 

 abbreviated, and the head-rays furcate and extended horizontally. (I) Four-rayed spicule of 

 Pachastrella primceva, Zitt., in which two rays are incipiontly furcate. (/) Simple trifid or zone- 

 spicule of Oeodites, sp. (g) Compound trifid or zone-spiculeof Oeodites planus, in which the head- 

 rays are furcate, {h) Simple trifid or zone-spicule of Geodites, sp. (?') Simple trifid or zone- 

 spicule, moniliform, Oeodites Wrightii. (j) Trifid or zone-spicule of Stelletta inclusa, Hinde, in 

 which the head-rays are recurved, (k) 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 furcate (Fig. 3, g), 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. o,j,Jc). Both rays and shaft are 



