MOLECULAR COALESCENCE. 



775 



this plan that the further increase takes place, by which such 

 larger concretions as are shown at a, a, are gradually produced. 

 The structure of these, especially when examined by Polarized 

 light, is found to correspond very closely with that of the small 

 calculous concretions which are common in the Urine of the Horse, 

 and which were at one time supposed to have a matrix of cellular 

 structure. The small calcareous concretions termed ' Otolithes,' 

 or ear-stones, found in the Auditory sacs of Fishes, present an 

 arrangement of their particles essentially the same. Similar con- 

 cretionary spheroids have already been mentioned (§ 507) as 

 occurring in the Skin of the Shrimp and other imperfectly-calcified 

 Shells of Crustacea ; they occur also in certain imperfect layers of 

 the Shells of Mollusca ; and we have a very good example of them 

 in the outer layer of the envelope of what is commonly known as a 

 ' soft Egg, ' or an ' Egg without shell, ' the Calcareous deposit in the 

 fibrous matting already described (§ 558) being here insufficient to 

 solidify it. In the external layer of an ordinary Egg-shell, on tbe 

 other hand, the concretions have enlarged themselves by the pro- 

 gressive accretion of Calcareous particles, so as to form a con- 



Fig. 412, 



Artificial Concretions of Carbonate of Lime. 



tinuous layer, which consists of a series of polygonal plates re- 

 sembling those of a tesselated pavement. — In the solid 'shells' of 

 the Eggs of the Ostrich and Cassowary, this concretionary layer is 

 of considerable thickness ; and vertical as well as horizontal sec- 

 tions of it are very interesting objects, showing also beautiful 

 effects of colour under Polarized light. — From the researches of 

 Prof. Williamson on the Scales of Fishes (§ 548), there can be no 

 doubt that much of the Calcareous deposit which they contain is 

 formed upon the same plan ; and it is probable that by a further 



