derived from the impulse of the tail. ‘They are em- 
ployed, also, in suddenly checking or stopping the 
motion, and giving it a more rapid acceleration.— 
But still the tail is the most powerful of the instru- 
ments for progression, being at once a vigorous oar, 
an accurate radder and a formidable weapon of 
offence.” 
This is a clear and brief description of the me- 
chanical function of fishes’ fins, but there is a com- 
prehensive view of the skeleton of fishes, which If 
would take, as more determinate yet. I do not ob- 
serve that physidlogists have given very prominent 
attention to it, and I should not have been led to 
notice it in the large way in which I refer to it now, 
had not, the possession of two hippocampuses, or 
sea-horses, as they are called, remarkably fixed my 
attention, specially, to the structure and power of 
the spinal column unaided or variously aided by fins. 
The spinal column is the essential instrument in 
the propulsion of fishes. It is the acting power re- 
gulating the accessory motion of the fins. In the 
several species lengthened out into the snake form, 
as in the eels and the cutlass-fish, the body is con- 
ducted entirely by the spine. The power of the 
posterior extremity is increased from side to side, 
by the bordering fin, adding to the skulling power 
of the column, but the essential instrument of motion 
ig the flexibility of the column itself. 
{ think the best and simplest way of considering 
the organization of a fish, is by the relation of: the 
spinal column and the accessory fins to modes of life 
