the Chalk, Greensands and Oolites. 185 



sponge of our rivers, and to many other parasitical species which are inhabitants 

 of the sea at the present period. Wherever the sponge had settled upon a shell, or 

 Echinus, this habit of coating the body it was based upon is strongly illustrated ; but 

 as the substance thus built upon was probably to a small extent immersed in the 

 silt or mud, we rarely find more than half or two-thirds of the surface enveloped, 

 and from this circumstance it is, that we detect in chalk fossils so many which are 

 more or less imbedded in flint ; and among these, instances are found, varying from 

 cases in which the sponge-gemmule had not long settled and commenced develop- 

 ing itself, to where it had grown to such an extent as to have exceeded many times 

 the size of the base, upon which it most probably at first settled. The great mass 

 of nodular chalk flints does not exhibit any indication of a base, except in those 

 instances where extraneous substances appear upon the surface ; but this may be 

 readily accounted for, when we consider, that probably, in the first instance, the 

 gemmule was attached to some minute fragment of a shell or other substance, and 

 that its further development took place while it was merely recumbent on the silt 

 or mud ; in the same manner as we occasionally find specimens of modern corals, 

 etc., without any apparent base, the original attachment having been built over, 

 after it had become separated or liberated. In one specimen of flint in my pos- 

 session, the sponge appears to have been torn or injured, nearly at its middle, as 

 a broad and distinct band or collar has been built round it, evidently with a 

 view of repairing the injury and strengthening the substance at the weakened 

 part. 



The perpendicular and oblique veins of flint, found in the chalk cliffs between 

 Brighton and Rottingdean, and described by Dr. Mantell, present precisely the 

 same internal characters as the tabular flint and the common tuberous nodules. 

 The external characters are also similar to those of the tabular flint. If we ob- 

 serve these veins in situ, we shall frequently perceive, that the whole of their sub- 

 stance is not of uniform density, but that there are often, near the middle of the 

 vein, parts where the two interior surfaces have not united, and that the spaces 

 are generally filled with chalk. If this chalk be carefully removed or dissolved by 

 diluted muriatic acid, the internal surfaces present the same appearances which 

 have been described as characterizing the exterior of the ordinary flint nodule 

 and the aspect of the whole is precisely such as we should expect to find, if the 

 two sides of a fissure in a rock were covered by our common freshwater sponge, or 

 one of similar habits ; and the two outer surfaces had been built towards each 

 other and had joined in some parts, while in others they had approached, but had 

 not united. The sides of these flint veins are not studded with numerous species 

 of ForaminifercB , like the under surface of the tabular flint, but from the position 



VOL. VI. SECOND SERIES. 2 B 



