42 



This is aided by the summer's increase of heat about harvest time. 

 It was shown that if uncongenial weather prevailed, such as wind 

 and wet, not only was the wheat stem often bent, preventing this 

 transfer of suitable material to the grain, but that the root threw up 

 fresh leaves and stems, being stimulated to fresh growth by the rain, and 

 thus the stimulus to the upper parts of the stem is withdrawn to the 

 great detriment of the crop. 



The lecture was illustrated by means of diagrams and specimens, 

 and the best and latest authorities had been consulted in its compilation. 

 Especial reference was made to the lectures on the physiology of plants 

 by Mr. S. H. Vines, publishel by the Cambridge University Press; and 

 a letter touching the matter of this lecture, which Mr. Dowker had 

 received from Mr. Vines, was read. 



IX. 



THE STRUCTURE AND GROWTH OF SHELL IN 



THE MOLLUSCA. 



Abstract of Faper 



READ BY 



MR. SIBERT SAUNDERS, 

 April 14th, 1890. 



The lecturer first pointed out the intimate connection which is found 

 to exist between the shell and the branchiaa or gills of the mollusca. 

 Shell in its simplest form exists in the common slug, and also in the 

 Tectibranchiate marine molluscs as a thin calcareous shield embedded 

 in the flesh of the animal, serving as a protection to the branchise ; 

 while in those cases where it forms a strong and solid covering, enclosing 

 the entire animal (whether bivalve or univalve), it is still intimately 

 connected with the breathing organ, the shell being in all case? formed 

 by the mantle, of which the gills are a specialised part. The two-fold 

 function of the mantle, in first, extending the margin of the shell by 

 applying to its edge fresh deposits of shell-tissue derived from its own 

 substance, and then in lining the whole interior with a different kind of 

 calcareous matter, deposited from its surface, was then referred to, and 

 the various kinds of shells were described. The internal lining found in 

 most shells presents a surface of perfect smoothness, and sometimes has 

 a high polish. This is said to be sub-nacreous, it not having the irri- 

 descent lustre of the true nacre, which is found only in a few families. 

 The pearly irridescence of true nacre is produced by the light falling on I 

 the outcropping edges of partially transparent plates forming wavy lines 

 on the surface of the shell. This striation is effected by the folding or 



