45 . 



Westphalia, (Zittel : iiber Coelop. Taf. VI, figs. 16 29) 

 and in the Eocene sand of Brussels (Rutot: op. cit. PI. Ill, 



fig- 9)- 



Quadrifid Spicules of Pachastrella. 

 Pachastrella, sp. 



(Plate III, figs. 24, 25). 



Simple quadrifid spicules in which three elongated arms 

 radiate at equal angles from a centre and form the outline 

 of a more or less elevated three-sided pyramid, from the apex 

 of which another arm generally shorter than the other three, 

 extends upwards. There are two very distinct forms of these 

 simple quadrifid spicules. In the larger (fig. 24) the three 

 arms form a very low pyramid ; in some examples they are 

 nearly in the same plane; -and the fourth arm, much shorter 

 than the other three, is nearly at right angles to them. The 

 other three arms are nearly equal, straight and pointed, and 

 in no instance which I have noticed are they bifurcated at 

 the extremities. Canals are shown in some examples, much 

 wider than those in similar spicules of recent sponges. These 

 spicules are abundant and very uniform in size. The average 

 length of the arm of a fair sized spicule is 1,035 mm - an d 

 its thickness near the centre is 0,09 mm. The vertical arm 

 has a length of 0,2 mm. 



In the smaller form of quadrifid spicule (fig. 25) the four 

 arms are nearly equal in length and they are disposed so as 

 to form a more elevated pyramid. The average length ot 

 each arm is 0,45 mm. and the thickness near the centre 

 0,045 mm - These spicules are also very abundant. 



From comparisons which I have made with specimens of 

 PacJiastrclla amygdaloides , Carter , and P. geodioidcs , Carter 

 (An. Mag. Nat. Hist., S. 4, Vol. 18, p. 406, PL XIV, figs. 22, 

 23) from the Atlantic, I find that there is a striking resem- 



