UPPER, OR FLINTY CHALK. gf 9 



Upon this subject, Professor Buekland remarks, " that the hollows 

 affording a mould for the formation of these bodies, are clearly the work 

 of some minute parasitical insect. The small aperture, the cast of which 

 now forms the projecting axis of each globule, was probably perforated by 

 this intruder, as the entrance to his future habitation. Having completed 

 this passage, and excavated at its termination a cell suited to his shape 

 and convenience, he appears by the aid of a dehcate augur or proboscis? 

 to have drilled many minute and ahnost capillary perforations into the 

 substance of the shell on every side around him ; taking care to leave 

 always partitions sufficient to support the thin external plate of the shell 

 which formed the roof of his apartment. Having exhausted all the 

 nourishment that could be procured in this manner with safety from the 

 vicinity of his first establishment, the insect appears to have emigrated, 

 and after working for itself a lateral passage to a considerable dis- 

 tance, to have formed a new settlement in the midst of fresh supplies. 

 In a recent oyster shell in my possession, this process has been carried on 

 to a great extent in the intermediate matter between two or three sets of 

 the pearly plates comprising it ; and yet without effecting the destruction 



casts, and their surrounding matrix, are composed of limestone, of a subcrystalline structure. 

 The specimen contains numerous casts and impressions of bivalve shells, univalves, madre- 

 porites, &c.; the forms of which are defined with much sharpness, and elegance. The cavities 

 left by the removal of the shells, are more or less filled with groups of globular bodies, and are 

 crossed by slender filaments, evidently moulded in perforations that existed in the recent 

 testaceae. 



As the constituent substance both of the shells, and casts, in the present fossil, were ori- 

 ginally composed of nearly similar materials, it appears difficult to explain by what agency the 

 one has been removed, while the other has remained uninjured. The following observations 

 of Mr. Parkinson, will illustrate the mode by which this operation has been effected. 



" Calcareous spar, exposed to the action of water, suffers through a long period but little 

 change or diminution ; while on the other hand, animal substances, such as shells, the crusta- 

 ceous parts of animals, and other bodies formed of an intermixture of animal membrane 

 or gelatin, with carbonate of lime, undergo a very rapid decomposition by the agency of water 

 alone, as is the case with dead shells, &c. on the sea and river shores. In these instances, the 

 animal membrane suffers resolution particle after particle, and layer after layer, and the car- 

 bonate of hme deprived of its cement and support, gradually separates and moulders away *." 

 Thus it has happened to the shells enveloped in the crystalline limestone before us ; while the 

 animal substances have been decomposed by the action of simple water, the investing matrix 

 has suffered no change, but perfectly retains the form and impressions of the animal bodies that 

 have passed away. 



* Org. Rem. Vol. ii. p. 173. 



F F 2 



