[On the supposed Sponge Spiculce in Flint. 467 



LV. — Gn the real nature' of the Minute Bodies in Flints, supposed 

 to be Sponge Spiculce. By William C. Williamson. 



To the Editors of the Annals of Natural History. 

 Gentlemen, Manchester, May Hth, 1846. 



An exceedingly interesting example of friable chalk, found at 

 Charing in Kent, having been placed in my hands by Dr. Man- 

 tell, I have been enabled by an examination of it under the mi- 

 croscope to correct an error into which I had fallen along with 

 other observers, as to the real nature of those minute fusiform 

 bodies, so common in chalk and chalk flints, and which have long 

 been regarded as spiculse of sponges. 



On examining a section of flint, even when it does not contain 

 the usual forms of Xanthidia and Foraminifera, there will gene- 

 rally be observed a number of small dark-coloured fusiform bodies, 

 which have been regarded by geologists as sponge- spiculae. The 

 same things are frequent in the soft chalk of Cambridge and Kent, 

 as well as at other localities. 



A slight inspection of the Charing chalk, where the organisms 

 are distinct and unmixed with amorphous matter, convinced me 

 that the half of the small atoms of which the pulverulent mass 

 was composed, consisted of bodies identical with those found in 

 flint. Observing them to be calcareous and not siliceous, as I 

 had expected, I was induced to make a more minute examination 

 of them, and soon became convinced that they were not the cal- 

 careous spiculse of sponges, but the separated prisms of disinte- 

 grated shell-structures, belonging to some genus of the group of 

 Margaritacece, as defined by Dr. Carpenter in his valuable Report 

 on the Microscopic Structure of Shells, published in the ^ Report 

 of the British Association ^ for 1844. 



The first thing that struck me in the Charing specimens was 

 their transverse lineation, a characteristic feature of shell-prisms, 

 but one which I have never seen in sponge-spiculse. Another 

 point of difference was, that instead of being round, as is usually 

 if not invariably the case with sponge- spiculse, they were angular, 

 having from four to six sides, which is also characteristic of shell- 

 structures. The correctness of the view I had taken was soon 

 settled by the discovery of a few specimens in which from two to 

 half a dozen prisms remained in their original contact, exhibiting 

 at one end the hexagonal reticulation so common in shell tissues, 

 and at the other the pointed contour, which characterized the 

 detached specimens. Even the latter portion presented a differ- 

 ent appearance from what we see in sponge-spiculae ; instead of 

 being thickest in the centre and gradually tapering away to each 

 extremity, these organisms are nearly of equal thickness through- 

 out a considerable portion of their length, and then taper off" 



