110 F. G. E. Pautard 



of crystalline bone salt have been recorded in invertebrates, in the Protozoa (Pautard, 

 1959), in the Arthropoda (Pautard and Trautz, in press) and in the Brachiopoda 

 (Klement, 1938). In the vertebrates, the familiar "phosphatic" structures of bone, 

 dentine and enamel have been supplemented by the calcified keratins, many of which 

 contain appreciable quantities of salt (Pautard, 1963). In plants, reports of deposits 

 of bone salts have been confined to the bacteria (Ennever, 1963) although there are 

 records of other forms of calcium phosphate in various types of heartwood. Calcium 

 oxalate in the form of intracellular crystals of diverse shapes and sizes is widely 

 distributed in plants, often, as in the case of some xerophytes, constituting over half 

 the dry weight of the plant. Reports of oxalate in animals are numerous, although 

 scattered. Crystals of this salt are often found in the invertebrates, particularly in 

 the Arthropoda, where deposits are found in insect egg shells and are resorbed during 

 embryonic development (for example, Moscona, 1948). In the vertebrates, calcium 

 oxalate in pathological stones and calculi is a familiar substance; in normal con- 

 ditions there is evidence that oxalate is found as a constituent of hard tissues (in fish 

 scales, for instance — Nishihara, 1954) and it is likely that small quantities are 

 present in soft tissues as well (Johnson and Pani, 1962). 



The present evidence, then, suggests that calcium in the form of carbonate, phos- 

 phate and oxalate is very widely distributed, can be found in one form to the 

 exclusion of others in animals and plants and often in individual species. Nowhere, 

 however, can we find evidence which confines any one salt to any one group. Instead, 

 we see a wide panorama of calcium salts with almost every variation and combi- 

 nation in different subjects. 



Structure 



At a molecular level, the structure of calcified tissues is resolved as a partnership 

 between inorganic deposits and organic substances. This partnership varies widely in 

 the amount and ordering of the mineral and organic phases, and we can find ex- 

 amples of different proportions of different salts in the same organic milieu and 

 similar proportions of similar salts in different organic milieu. There are wide 

 variations, too, in the order and crystallinity of the two phases. The three principal 

 calcium salts can be found in crystalline and non-crystalline states in organic environ- 

 ments varying from frankly fibrous proteins and polysaccharides to diffuse inde- 

 terminate stroma with no apparent structure. 



Mineral phase 

 While chemical assay of cations and anions can give us figures from which we can 

 calculate plausible structures, the best criterion of a salt is its characteristic X-ray 

 diffraction pattern, which results from an exact arrangement of atoms in a lattice 

 and thus relates the components in a positive geometric way. Unfortunately, X-ray 

 diffraction methods have limitations, particularly in biological subjects, and these 

 are often overlooked in studies of mineral deposits. In the first place, the usual pro- 

 cedures associated with powder analysis, especially where organic enclosure may 

 prevent equilibrium between different types of salt, are not quantitative and it is 

 not profitable to relate the crystallographic evidence to the chemistry. Again, a 

 crystal ceases to give an X-ray diffraction pattern below a certain limiting size 



