UPPER, OR FLINTY CHALK. 169 



open extremities of short transverse tubuli ; stirps fixed by radical pro- 

 cesses. 



So numerous are the accidental varieties of form assumed by the fossil 

 remains of this species, that it is difficult to distinguish them correctly, 

 without the assistance of an extensive suite of specimens. This circum- 

 stance is partly attributable to the various states of expansion and contrac- 

 tion, in which the originals were introduced into the mineral kingdom ; 

 and partly to the mode in which their remains are occasionally preserved. 



The specimens enveloped in flint, are usually of a cyathiform, or turbi- 

 nated shape (vide Tab. x.) ; while those imbedded in chalk, are more or 

 less expanded in the form of a broad circular disk, as in Tab. xiv. The ex- 

 ternal surface is composed of cylindrical fibres, that extend in a radiating 

 manner from the centre or base, to the outer margin, and by frequently 

 subdividing and anastomosing, constitute a reticidated integument capable 

 of very considerable contraction and expansion. Vide Tab. xii. figs. 1, 2. 

 Tab. xiii. figs. 2, 3, 4, 5. Tab. xiv. fig. 2. 



The fibres are solid*, and when viewed through a lens, exhibit a 

 porous structure, bearing considerable resemblance to dried sponge. The 

 meshes, or interstices between the fibres, are narrow and elongated in the 

 specimens that are expanded, but very irregular in those which are cor- 

 rugated by contraction. In some instances slender transverse filaments 

 extend from one fibre to another, by which the entire plexus is more firmly 

 connected together ; these are represented in the magnified sketch, fio-. 6, 

 Tab. xiii. The surface of the interior, or funnel-shaped cavity, is studded 

 with small perforated tubercles, or papillae, the open extremities of short, 

 straight, cylindrical tubes, that arise between the fibres of the external 

 integument, and passing in a transverse direction, terminate on the inner 

 surface. Siliceous casts of these tubuli, are frequently observable in flints 



* A different opinion is entertained by Miss Benett, who assures me, that in several Wilt- 

 shire specimens in her possession the fibres are hollow. I have not, however been able 

 to d«tect such a structure in any of the numerous examples that have been submitted to my 

 examination. 



