42 INVERTEBRATE PALAEONTOLOGY 



shells or tests that were not filled by matrix, such as 

 many Brachiopods and Irregular Echinoids. In these 

 cases the sparry material, usually calcite but sometimes 

 quartz, projects into the cavity and may even fill it. In 

 Terebratulids from the Faringdon Greensand, stages 

 in this incrustation can be traced, from a crystalline 

 coating of the valves and brachidia to a complete 

 internal mould, through whose transparent substance 

 the "loop" can be seen. This style of infilling is well- 

 adapted for preservation of the delicate internal ap- 

 paratus of the Telotremata ; and, thanks to the methods 

 of preparation devised by the Rev. Norman Glass, does 

 not prevent investigation of its structure. 



Less commonly similar incrustation may develop 

 over the external surface of a fossil. This normally 

 obscures the finer details of ornament ; but, in the case 

 of Echinoids, it emphasizes the plate-structure by its 

 crystalline relation to the ossicles of the test. Incrusta- 

 tion is normally present on the Echinoids of the Aptian 

 of Faringdon, and sometimes occurs on those of the 

 Chalk. In fossils from the former deposit an outer 

 film of iron-oxide coats the excrescent calcite, and both 

 layers may often be chipped off, leaving the unchanged 

 test exposed. 



(II) CASTS AND MOULDS (PI. Hi. fig. 2; PL iv. figs. 5 

 and 6; PL v. fig. i; PL vi.) 



In permeable rocks it is usual to find that fossils 

 differing from the matrix in composition have been 

 entirely removed in solution. If their removal has been 

 postponed until the surrounding rock attained some 

 rigidity, and if the texture of the matrix is sufficiently 

 fine, the scars representing the missing fossils will retain, 

 in negative, all superficial details of structure and orna- 

 ment. Such negatives are termed "casts" when they 



