43 



plication of the animal membrane deposited by the mantle, by which 

 the nacre is secreted. The folds, lying more or less obliquely to the 

 general surface, and being arranged with considerable regularity, cause 

 the diffraction of light upon which the irridescence depends. Similar 

 folds of basement-membrane exist in sub-nacreous shells, but the 

 arrangement of the lines lacking regularity, no pearly lustre is produced 

 on the surface. 



Living shells are covered with an outer coat of animal matter — the 

 epidermis or periostracum. In some cases tliis is so thin and transparent 

 as to be scarcely perceptible. In others it is a dense homy skin. 

 Sometimes it takes the form of a silky woven material. It serves 

 to protect the shell against the erosion of chemical agents. Its 

 appearance during the life of the animal is very different from that 

 which it presents in " dead " shells, where it soon shrivels up and 

 peels off. Like the outer layer of shell, the periostracum is formed by 

 the glandular margin of the mantle. Some of the glands contained 

 in the thickened margin of the mantle secrete colouring matter. 

 The pigment so furnished is mixed with the calcareous additions made to 

 the edge of the shell, and thus the outside of the shell becomes nainted 

 with coloured spots or lines, which vary in different species according to 

 the position of the colour glands, and also accoi'ding to the intervals 

 between the periodical secretion of colour. The spines, plates, and 

 ridges, seen in many shells are produced by the periodical protrusion 

 of certain portions of the edge of the mantle. The spines thus 

 secreted may be separated from the next row by a considerable space, 

 owing to the intermediate deposition of a greater or less amount 

 of shell substance presenting a plain or rigid sui-face. Some remarkable 

 examples of the repair of broken shells and the modification of form, 

 where necessary, were described and illustrated by specimens. Examples 

 of all the kinds of shell refeiTcd to in the paper were exhibited, and the 

 cellular and prismatic structures demonstrated by the microscope. 



X. 



SANDPIPES. 

 Abstract of Paper 



BY 



Mr. ARTHUR S. REID, M.A., F.G.S. 



Read before E. K. N. .S., Monday, April 13th, 1891. 



The Author having pointed out the frequent occurrence of sandpipes 

 in limestone districts, alluded especially to those of the North and South 

 Downs ; he then gave a description of the gradual formation of a sandpipe 



