Prof. Ansted on the Zoological condition of Chalk Flints, ^c. 245 



same spongeous origin as tlie flint on wliicli it reposes. The siu*- 

 faces however within and without this shell are not on the same 

 level, but the flinty matter has reached considerably higher on 

 one side than on the other, and has entered the shell on the side 

 opposite to that on which it has the highest surface. This latter 

 fact is clear from the perfect condition of the shell. But we can- 

 not suppose a thick pulpy fluid to have entered the shell at its 

 lower part and have risen in it on the opposite side above its ge- 

 neral level, although there is no difficulty in conceiving that a 

 sponge so entering an inclosed space should work its way upwards 

 in the endeavour to reach the open water. 



With regard also to the surface of the flint and its partially 

 calcareous appearance, it is readily explained, if we suppose the 

 flint to have been originally a sponge ; in that case a deposit of 

 calcareous mud taking place upon it would penetrate only to a 

 very small depth, while the interior space would remain filled with 

 pure water. By far the greater portion of the sponge would thus 

 be preserved from the operation of other agents while the pro- 

 cess of silicification was going on, provided it is the case (as we 

 have every reason to suppose) that the silex forms readily on such 

 horny organic matter as is found in all sponges. 



The conclusions thus arrived at as to the origin of tabular flint 

 are greatly strengthened when we consider the nature of the com- 

 monest chalk fossils that occur silicified, and the peculiar appear- 

 ances often presented by chalk flints. 



By far the most abundant of those fossils in the chalk which 

 attain to any size are the remains of spongy Zoophytes, kno^Mi 

 under the generic names of Ventriculites, Choanites, Paramoudra 

 and Polypothecia. 



The Ventriculite is found of various shapes and sizes, and its 

 original texture was evidently soft, and yielding readily to press- 

 ure. It appears to have possessed externally a reticulated sur- 

 face, the inside being porous and bearing considerable resem- 

 blance to dried sponge. Specimens occur in every intermediate 

 form between that of a simple elongated cone and a flat circular 

 disc, the thickness of the sides being considerable when the cone 

 is short, thinner when more extended, and thinnest when com- 

 pletely expanded. 



The Choanites difi'er from Ventriculites by the possession of a 

 circular opening in the upper part, which continues gi-adually 

 diminishing to near the base. The general proportions are also 

 somewhat difiierent and the dimensions smaller. 



The Paramoudra, or Potstones, are only common in particular 

 localities ; but there attain a considerable size, measuring from one 

 to three feet in height, and about half that in diameter. They 

 are chiefly found in the North of Ireland and in chalk-quarries 



