210 DR. J. MURIE ON THE ANATOMY [Feb. 14, 
the same manner as a boat is sculled by means of an oar over the 
UOT sy a/ists tats Seat When desirous of proceeding at a greater rate, 
the action of the tail is materially altered; instead of being moved 
laterally and obliquely, it strikes the water with the broad flat sur- 
face of the flukes in a direct manner upwards and downwards.” 
The first part of Beale’s description seems to me pretty well to 
explain the manner of action of the tail in gentle forward movement ; 
for if we take it as a mere dynamical agent, then, according to the 
law of forces, the lobes of the tail, striking the water between the 
horizontal and vertical, would cause the body of the animal to move 
in the diagonal of these*. We have then to consider if the lobes do 
strike the water in this oblique manner. 
In illustration of its probability, I may mention that in this spe- 
cimen I noticed that as the lobes of the tail began to dry by the 
heat of the atmosphere they assumed opposite concavo-convex curves, 
so as to produce a very close resemblance both in curve and angle 
to the blades of a screw propeller. If this is what usually takes 
place in the movement of the tail, then one can readily understand 
that in the act of striking downwards and upwards with but a very 
slight lateral movement the water would receive an oblique stroke 
between the horizontal and vertical ; in fact it would produce a more 
or less scooping, spiral, screw-like action, the consequence of which 
would be movement in the diagonal of the parallelogram of the 
resisting forces, from side to side, above downwards and below up- 
wards. 
As a further demonstration that something like this occurs, I may 
state that I observed in the mode of progression of the Porpoise 
lately exhibited in the Society’s Gardens a movement in the tail more 
or less resembling this. As the creature swam horizontally, and 
within a short distance of the surface, it struck the water up and 
down, but with an inclination of the lobes to the one or other side, 
adding to the obliqueness of the stroke by rolling its body slightly, 
so that the tail struck the water rather slantingly than directly up 
and down. When simply raising itself towards the surface (as, for 
instance, in breathing), or in descent, then the stroke seemed more 
flat, and the body, along with the tail, bent in a curve according as 
the movement was elevation or depression. 
In Physalus, the arch of the mouth, both antero-posteriorly and 
transversely, is much less than in the true Balene ; the length of 
the whalebone is also correspondingly shorter. In our specimen of 
P.antiquorum, the transverse curve of the former was about 72 inches 
at its widest, that is including the bony palate and the filaments of 
whalebone on either side; but towards the beak it became flattened 
and so narrowed that the chord of the arch was but 10 inches. 
The free palate itself in the middle of the mouth had a breadth of 
12 inches. 
The outward exposed parts of the baleen zz situ on one side mea- 
sured from the beak to the angle of the mouth in a direct line 126 
* See article ‘“ Motion,” ‘ Cyclop. Anat. and Physiol.,’ vol. iti. p. 438. 
