arms of an even thickness throughout has also been found in 

 the Irish Chalk. (Wright: op. cit. PL III, 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. 3136. 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 0,40 mm ; average 

 width of head 0,112 mm. 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 0,607 mm. an< ^ the width of the head is O,i8mm. 

 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 0,5 17 mm. 

 and the expansion of the head rays 0,45 mm. The shafts of 



