64 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Dec. 4, 



Ventriculite-structure, the skeleton is considered to be composed. On 

 examining one of the above crosses, it is seen that its radii start 

 from their common centre as very fine filaments indeed ; but after 

 pursuing their course for a quarter of their entire length, they 

 undergo a sudden enlargement in diameter, and continue uniformly 

 of this increased size to their terminal anastomosis. At the point 

 of rather abrupt change in the diameter of the radius two other 

 fibres are given off, one on each side, which pass backwards at an 

 angle of about 45°, to join similar fibres similarly passing from the 

 other arms of the cross ; in this manner, about the centre of the cross 

 under consideration, is formed a roughjsquare, which represents the 

 octahedral stays in section (fig. 1). This description applies equally 

 to the sections figured of Toulmin Smith's Ventriculites in flint, and 

 to those of the Cambridge forms in phosphatic material ; and conse- 

 quently, according to the admitted diagnosis of the family, there can 

 be no doubt that our Cambridge fossils must be included in the same 

 group with those of the Chalk, whilst a detailed examination has 

 enabled me to identify, from our formation, forms belonging to some 

 four of T. Smith's species ; and there are others in my possession 

 which I hope to determine when time allows. 



Modes of Fossilization. — The fibre of our Cambridge Ventriculites 

 presents itself under four different conditions of preservation. 



(i.) Simple opaque fibres, of which the distal radial fibres are from 

 T25I)" ^° ttkju" ^ n diameter. Generally insoluble in hydrochloric 

 acid. 



(ii.) The simple fibre of No. i. is frequently surrounded by trans- 

 parent colourless walls, from ^(nny" to 20V0" in thickness, and gene- 

 rally well defined on their inner and outer circumferences. They give 

 no colours with polarized light, but shine brightly on the dark 

 ground produced by crossed prisms. Under a high power they seem 

 to be made up of a number of crystals placed perpendicularly to the 

 surface on which they are set. In hydrochloric acid they dissolve, 

 but not so readily as the surrounding coprolite. The opaque fibre 

 they enclose is next to insoluble in hydrochloric acid ; it remains as 

 an opaque yellowish-brown rod after the colourless walls have been 

 altogether dissolved away. 



(hi.) The opaque fibre of No. ii. has entirely disappeared, leaving 

 only the colourless walls which surrounded it ; these now form a 

 cylindrical tube, measuring usually from g^-jj" to - 4 fa" in diameter 

 in the distal radii, and having a central hollow cavity of from y^s u" 

 to -g-fj- 17 " diameter (fig. 1). 



(iv.) The colourless walls and opaque fibre have both disappeared, 

 and their place is occupied by a finely granular material, appearing 

 in section as a broad band, of variable diameter, frequently from 

 -g^-jj" to -j^-q" across, generally of a brown colour, but sometimes 

 pale green from infiltrated glauconite. With polarized light it 

 appears strongly luminous on the dark ground produced by crossed 

 prisms. In hydrochloric acid it is far less soluble than the adjacent 

 coprolite. 



In some sections of the Cambridge Ventriculites, simple opaque 



