1856.] BUIST MOLECULAR CHANGES. 11 



vindication of the full amount of the value of the law iu the interpre- 

 tation of fossil remains, as defined by the illustrious founder of Palae- 

 ontology. 



EXPLANATION OF PLATE I. 



Stereognathus ooliticus. 



Fig. 1. Side view of the portion of lower jaw and three teeth, with tlie matrix : 

 nat. size. 



Fig. 2. Upper view of the same : nat. size. 



Fig. 3. Outer side of the middle one {b) of the three teeth : magnified 8 dia- 

 meters. 



Fig. 4. Inner side of the same, similarly magnified. 



Fig. 5. Upper or grinding surface of the same tooth, similarly magnified. 



[The letters are explained in the text.] 



November 19, 1856. 

 William Downing Biden, Esq., was elected a Fellow. 

 The following: communications were read : — 



'O 



1. On the Occurrence o/ Crystallization in a Stucco-Casting. 

 By G. BuiST, LL.D., F.R.S., F.G.S. 



In 1847-48, having been much engaged in plaster-casting at Bombay, 

 in the spring of the latter year I had thrown out a multitude of 

 useless fragments, when, on the 28th and 29th of March, a furious 

 thunder-storm and fall of rain occurred at the setting in of the mon- 

 soon. A week or so afterwards, as soon as the weather slightly 

 cleared up, what was my astonishment to find nearly all the frag- 

 ments presenting crystals of semitransparent selenite on their surfaces ! 

 These, on examination, were found partly imbedded in the earthy 

 mass of stucco, in part protruding above its surface. When the 

 specimens were taken indoors, the crystallization extended itself, and 

 the whole mass became covered over with fine pearly spicula, like 

 the crystals of the finest actinolite, only not radiating from points. 



The stucco was prepared from a hard crystalline gypsum brought 

 from the Persian Gulf; deprived of its water in the usual way, by 

 being heated a little short of redness, then ground and sifted, and 

 treated in the customary fashion. It was so absorbent that the rain 

 disappeared from its surface almost as rapidly as it fell, so that the 

 .crystallization could not have arisen from saturation, in the ordinary 

 sense of the term, or deposition from a menstruum. 



I am not prepared to attach any influence to the thunder-storm : 

 it was one of almost unprecedented violence and duration, and its 

 occurrence seems worth mentioning, because on no future occasion 

 was I ever able to obtain like results from exposure of stucco to the 

 weather : there might at the same time have been peculiarities iu the 

 specimens thus transformed, of which I w^as unconscious. 



The crystals are semitransparent, well formed, and deeply grooved 

 cOxially along their faces. They are so perfectly unhke the material 



