from the Middle Lias of Northamptonshire. 357 



Middle Lias, in the zone of Ammonites sjn'natus, at King's 

 Sutton, Northamptonshire. Associated with the sponges are 

 numerous specimens of well-preserved Foraminifera, Corals, 

 Mollusca, and Polyzoa ; these latter have already been 

 described by Mr. Walford (Qaart. Journ. Geol. Soc. vol. xliii. 

 1887, p. 636). 



II. Detached Calcisponge Spicules in the Upper Chalk 

 of Surrey. (PI. XVII. fig. 10.) 



In some mountings of the finer material of the Upper Chalk 

 (zone of Micraster) from Croydon and Sutton, Surrey, there 

 are, in addition to the common Foraminifera! species of T'ex- 

 tularia^ Glohigerina, &c., some minute three- and four-rayed 

 spicules, very similar in appearance to those of ordinary 

 Calcisponges. The spicules are of calcite, their forms are 

 fairly complete, but their surfaces are rough and uneven, as 

 if covered by the finest particles of the Chalk. The rays are 

 conical, with blunt terminations ; they vary from "04 to '13 

 millim. in length and from '007 to '02 millim. in thickness. 

 Some are regular forms with rays equal in length ; in others 

 the rays are unequal. Beyond some rod-like fragments of 

 the same thickness as the three-rayed forms no other spicules 

 are present in the material. As these spicules correspond in 

 form, size, and mineral structure with those of Calcisponges, 

 it seems reasonable to conclude that they are detached from 

 sponges of this group. The only other inference is that they 

 may be spicules of siliceous Tetractinellid sponges which 

 have been replaced by calcite. But against this supposition 

 is the fact that even the larger forms of true siliceous spicules 

 are very rare in the Chalk of these areas (unless included in 

 the cavities of flints) ; they have been dissolved, leaving 

 empty moulds in the chalky matrix. Further, in these spicules 

 the three facial rays are approximately in the same plane, the 

 same as those of Calcisponges generally, whereas in the 

 Tetractinellid Calthrops spicules the rays are generally dis- 

 posed in the form of a tripod. It would also be very unusual 

 to find such very small detached forms which had undergone 

 mineral replacement. As Calcisponges of the genus Elasmo- 

 stoma are found in the Chalk of Kent, the occurrence of 

 detached spicules might have been anticipated ] but they do 

 not appear to have been noticed previously. 



