arms of an even thickness throughout has also been found in 
the Irish Chalk. (Wright: op. cit. Pl. If, fig. 5.) Dr. Bower- 
bank has also delineated a spicule from the existing sponge 
Euplectella aspergillum Owen, (Monogr. Brit. Spong. Vol. I, 
p. 257, Pl. VIII, fig. 174) which somewhat resembles the 
larger spicules from Horstead, but it is only about one-fourth 
the length and the central inflation is not so pronounced. 
Anchoring Spicules of Hexactinellids. 
(Plate I, figs. 31—36. Plate V, figs. 27.) 
Mostly small spicules with elongated shaft and variously 
shaped head from which 4, and in one instance 6 rays or 
barbs project backwards. There are present in the Horstead 
flint several different forms of these anchor spicules which are 
all characterized by a more or less inflated head and a 
shaft which is short in all the examples preserved, but may 
originally have been extended. One of these forms (figs. 34, 
36) has a very slender shaft and the head shaped like a four- 
sided pyramid from the base of which four minute rays 
project backwards. The shaft in some of these spicules 
appears to have been when complete short and pointed and 
it is doubtful if these anchor spicules really belong to Hexac- 
tinellid sponges. Length of these spicules 0,40mm,; average 
width of head 0,112mm. In another form of spicule (figs. 33, 
35) the head has the form of a cone, from the base of which 
four or six minute rays project backwards at an acute angle 
with the shaft. An example of this form (fig. 33) has a 
length of 0,607mm. and the width of the head is 0,18 mm. 
A third form (figs. 31. 32) has a more robust shaft than the 
others, and the head is obtusely rounded with four curved 
arms radiating from it like the ribs of an umbrella. In one 
specimen, the length of the incomplete spicule is 0,517 mm. 
and the expansion of the head rays 0,45mm. The shafts of 
