I0i M. D'Orbigny on the Comparative Position of 



In th^ above eight experiments between the maximum and minimum 

 rise of the piston, there is a diiFerence of five-tenths of an inch. How 

 this difference arises I am at a loss to know. A difference of about the 

 same extent existed throughout all my repetitions of experiments. I have 

 bestowed a great deal of labour and attention to find out how this differ- 

 ence arises, and I am satisfied that it has not its origin from any defect 

 in my apparatus or arrangements. I believe it arises from the difference 

 of strength that may exist between the different sparks of electricity with 

 which the gases were exploded, as it was with the spark from a Leyden 

 jar with which I exploded the gases. I intend making a set of experi- 

 ments, in order to ascertain this point. 



When commencing those experiments, I attempted to explode the 

 gases by the spark which is formed when contact is broken between the 

 wires of a battery ; but I found that this spark, although very bright, 

 would not explode the gases. The battery which I used for this pur- 

 pose was composed of eight narrow cast-iron troughs, with a plate of 

 zinc in each measuring twelve inches square. I remain yours truly, 



Jas. Johnston. 



Willow Park, Greenock, 15th March 1843. 



Some considerations regarding the comparative normal position 

 of Bivalve Molluscous Animals. By M. Alcide D' Orbigny. 



After all that has been written on the position of a bivalve, we would 

 naturally expect that men of science should be agreed as to this import- 

 ant point ; such, however, is by no means the case, and the examination 

 I propose to make of the different methods employed will prove this but 

 too clearly. 



Linnaeus, Bruguiere, Lamarck, and Bosc, termed the side on which is 

 the ligament, the base, and according to them the gaping portion of the 

 valve is above ; it is the upper side. 



M. de Blainville considers a bivalve in a position diametrically the op- 

 posite of that adopted by the authors just quoted; thus, the upper side 

 of Lamarck becomes the lower side of De Blainville. 



M. Deshayes follows neither the one nor the other of these methods ; 

 he reverses the shell altogether, so as to place the side where the tubes 

 are below, and the side where the mouth is above. According to him, 

 the side of the mouth is anterior, the side of the tubes is posterior ; the 

 length, however, is the same as that adopted by M. de Blainville. 



An examination of these various systematic positions, with reference 

 to the normal position of bivalves, has proved to me that they are all 

 more or less faulty. Every one who studies the shells in their natural 

 position, finds that a Solen, a Mya, a Pholas, and even a Venus, invariably 

 have their tubes projecting upwards to the surface of the sand, from the 



