66 



BRITISH FOSSIL SPONGES. 



3. Lithistid spicules, with four or more rays proceeding from a common centre, 

 or irregularly branching forms. 



4. Hexactinellid spicules, with six rays, forming three axes which cross each 

 other at right angles. 



5. Octactinellid spicules, with eight rays, six of which are in one plane and the 

 other two form an axis at right angles to the plane. 



6. Heteractinellid or polyaxile spicules, with a variable number of rays extending 

 from a common centre. 



Various modifications of the typical forms of these different groups are of 

 frequent occurrence. These may arise either by an increase in the number 

 of rays by furcation, or by a suppression of one or more of them. Thus, in 

 tetractinellid spicules subdivision takes place oftentimes in one or more of the 

 principal rays, and the same thing rarely also occurs in hexactinellid spicules ; in 

 these, however, suppression of one or more of the normal six rays frequently 

 happens. Again, the spicular rays may become modified so as to form an entire 

 or lobate disc, as in the dermal spicules of some lithistid Sponges. 



Fig. 2. — Different forms of fossil siliceous monactinellid spicules from Upper Greensand, Chalk, and 

 Tertiary strata, (a) Curved, fusiform, acerate spicule, showing the axial canal open at both ends. 

 (J) Acerate spicule with minute constrictions. (c) Vermiculate nodose acerate from Scoliorhaphis 

 cerehriformis, Zitt. (rf) Vermiculate acerate, smooth. (e) Acerate, acutely pointed. (/) 

 Acerate, probably immature, having the axial canal open throughout, (g) Simple curved 

 cylinder, with axial canal completely enclosed, (h) Cylinder, microspined. (V) Moniliform 

 cylinder, Monilites Ealdonensis, Carter, (j) Conical spicule. (Tc) Smooth, curved, acuate spicule. 

 (I) Pin-shaped or spinulate. (mi) Spinulate, showing the axial canal. (n) Acuate spicule, 

 microspined. (o) Tibiella, fusiform, with inflated extremities, (p) Moniliform acuate. (?) 

 Bihamate, Esperites Ealdonensis, Carter, (r) Bihamateor clasp-hook (flesh-spicule). (s) Bispatu- 

 late (flesh-spicule). (f) Equianchorate (flesh-spicule). 



1. Monactinellid Spicules. — The typical character of the spicules of this group 

 is a simple unbranched axial canal, which generally extends throughout the length 



