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 furecation, 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. 
Fie. 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. 
(2) Acerate spicule with minute constrictions. (c) Vermiculate nodose acerate from Scoliorhaphis 
cerebriformis, Zitt. (d) Vermiculate acerate, smooth. (e) Acerate, acutely pointed. (/) 
Acerate, probably immature, having the axial canal open throughout. (yg) Simple curved 
cylinder, with axial canal completely enclosed. (h) Cylinder, microspined. (7) Moniliform 
cylinder, Monilites Haldonensis, Carter. (j) Conical spicule. (4) Smooth, curved, acuate spicule. 
(J) Pin-shaped or spinulate. (m) Spinulate, showing the axial canal. (m) Acuate spicule, 
microspined. (0) Tibiella, fusiform, with inflated extremities. (py) Moniliform acuate. (¢) 
Bihamate, Esperites Haldonensis, Carter. (r) Bihamate or clasp-hook (flesh-spicule). (s) Bispatu- 
late (flesh-spicule). (¢) Equianchorate (flesh-spicule). 
1. Monactine.tip Sprcutus.—The typical character of the spicules of this group 
is a simple unbranched axial canal, which generally extends throughout the length 
