20 



Horstead flint, and they are in a much more perfect state of 

 preservation. 



In describing these Horstead spicules, I propose to begin 

 first with the simple uniaxial forms and then pass on to the 

 quadriaxial spicules and the more complicated lithistid and 

 hexactinellid forms ; following the systematic arrangement which 

 is given by Professor Zittel in his « Beitrage zur Systematik der 

 Fossilen Spongien*. 



Monactinellidse , Zittel. 



To this order belong those sponges whose skeleton con- 

 sist only of those spicules which possess a single unbranched 

 interior canal. But as simple monaxial spicules are also 

 common to other orders of sponges as well, the question of 

 determining the proper relationship of these spicules, when 

 detached, as in the specimens under consideration, is some- 

 what involved. As the only method available, I have com- 

 pared the simple uniaxial spicules in this deposit with those 

 which are present in recent and fossil sponges of the order, 

 as well as with the uniaxial spicules which are found in con- 

 nection with the distinctive spicules of Geodia and other genera, 

 and I have come to the conclusion that the greater number 

 of the uniaxial spicules in this deposit more probably belong 

 to Geodia and thus formed part of the Skeleton Spicules of 

 Tetractinellid sponges, under which I shall include them. 

 Those which appear to have belonged to Monactinellid sponges 

 are not very numerous. 



Acuate Spicules. 



(Plate I, figs. 10—15.) 



Simple acuate spicules, somewhat resembling pins, but 

 with less prominent heads, curved, thickest at one extremity 

 and gradually tapering to the other, which in some instances 

 is obtusely pointed. Originally smooth, though now with 



