from the Middle Lias of Northamptonshire. 353 
rayed spicules of varying dimensions, which, for the most 
part, are indiscriminately intermingled together. Most of the 
acerate spicules are nearly straight, approximately cylindrical 
rods, with styliform, slightly inflected extremities (figs. 9 a, 
b). Of the largest of these I have not met with a complete 
form; the longest fragments measure ‘43 millim. in length 
and from *005 to ‘01 millim. in thickness. Other acerate 
spicules are straight or curved and either fusiform, gradually 
tapering to an acute point at both ends (figs. 9, d, e), or nearly 
of an even thickness for the greater portion of their length, 
and then terminating acutely (figs. 9, c, g). These latter 
range from 09 to °29 millim. in length and from ‘0037 to 
“007 millim. in thickness. The rod-shaped spicules appa- 
rently form the majority in this species ; some of the longer 
forms are disposed either parallel with the wall or in an oblique 
direction, so that their distal ends slightly project beyond 
its general surface. 
Of the three-rayed spicules (fig. 7) some are regular in 
form, that is with the rays of equal length, in others the 
rays appear to be unequal; but as one or more are usually 
broken, it is not easy to determine how far they may have 
been similar originally. The rays are smooth, straight, or 
rarely with a slight curvature, and very gradually tapering 
to an acute point. Sagittate forms appear to be absent. 
There is a great difference in the size of these spicules; in a 
small specimen the rays are not more than ‘03 millim. in 
length by ‘004 millim. in thickness, whilst the rays of a large 
spicule are ‘26 millim. in length and ‘01 millim. in thickness 
at the base. In the four-rayed spicules (fig. 8) the facial or 
plane rays resemble those of the three-rayed forms and the 
additional apical ray appears to be usually shorter than the 
facial rays, and in some it is distinctly more robust and some- 
what abruptly pointed. The rays in some instances are also 
decidedly elliptical in section at their bases. The three- and 
four-rayed spicules are, so far as can be ascertained, irregu- 
larly intermingled with one another and with the acerate 
spicules in the structure of the wall, and no special arrange- 
ment either on the dermal or cloacal surfaces is apparent ; 
but it is quite possible that the stout apical ray of the four-’ 
rayed spicules may project into the cloacal cavity, though not 
now recognizable in position. 
Of the canal-system in the walls of this species very little 
can be ascertained; there are here and there minute circular 
holes on the outer surface, which may be apertures of incurrent 
canals, and in fractured portions of the wall there are traces 
of anastomosing canals; also on the inner or cloacal surfaces 
