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, 

 h). 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, c?, 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 J tliat 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 difierence 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 



