PROCEEDINGS OF THE SOCIETY. 743 



work in connection with shell structure. Ideas had changed with regard 

 to the nature of shell formation, and he thought very few people would 

 now be found to hold that shell was formed by the calcification of the 

 cells of the mantle membrane. It was formed, no doubt, by the cells, but 

 not out of them ; each cell formed its own portion, and each layer as it 

 was thrown off contributed its traces to the markings of the shell. Then 

 the tubular structure, which they sometimes heard about, was purely the 

 result of the invasion of the shell by fungoid growths. Taking as an 

 illustration the common Pinna shell, Prof. Stewart, by means of draw- 

 ings on the board, explained how its structure was built up. He thought 

 that though the sections usually shown under the Microscope, both 

 vertical and horizontal, were useful and interesting in their way, they 

 must be supplemented by other methods if a correct idea was to be 

 obtained, the best way being to break the shell and to examine the 

 fractured surfaces. In the Pinna the nacreous layer only extended over 

 a small portion of the interior of the shell, aud if a piece was cut out 

 showing the growing edge of the nacre, and this was examined with the 

 binocular, it would be found to show the transition between the nacre 

 and prismatic layers, and would confirm the desirability of giving atten- 

 tion to the free and undisturbed structure. Whether his preparations 

 were now in good condition to show this he could not say, as theyihad a 

 tendency to effloresce if exposed to damp, but if good he should be very 

 pleased to bring them up to the next Meeting for exhibition. When 

 carbonate of lime was associated with colloid material, it had a tendency 

 to form spheres, and if they examined the shell of a large oyster such 

 as came up from the deeper portions of the sea, they would find these 

 spherical bodies in some of its layers. The value of the fracture in 

 ascertaining structure was particularly evident when they were examin- 

 ing the shells of the Gastropods — the formation of which was shown 

 by drawings on the board. He did not, however, wish it to be thought 

 that he despised a balsam mount or a ground section, but these should 

 be supplemented by other ways. The reproduction of the shell when 

 broken was also very remarkable and well worth observing. If a large 

 snail was taken and a piece was broken out of the shell so as not to 

 injure the animal inside, they would find that in the course of seven or 

 eight hours the hole would be closed by a thin film of organic matter. 

 In a few hours more little spheres and crystals would appear on this, 

 scattered at first sparsely but soon becoming thicker, until at last the 

 layer became solid, and finally lined by a layer similar to that of the 

 uninjured shell. When called upon by the President to say something, 

 he felt bound to obey ; but though there was nothing very new in what 

 he had said, he hoped some one might have got a hint or two which 

 would be of use in following up the subject. 



The President was sure the Fellows present were very much indebted 

 to Prof. Stewart for his instructive demonstration. 



The hearty thanks of the Society were then, upon the motion of the 

 President, voted to Prof. Stewart for his remarks, and to Mr. B. B. 

 Woodward for bringing up his very interesting collection. 



