56 Geological Society. 
as I gave in my last latter, is sufficient to prove the theorem 
to be untrue. 
If the velocity common to all parts of a fluid must be elimi- 
nated before any use is made of the equations of fluid motion, 
how is this velocity to be defined? By conceiving a velocity 
equal and opposite to that of any arbitrary particle impressed 
on the whole mass, we may get an infinite number of such 
velocities. . 
Professor Challis has brought it forward as an objection to 
the cases in which udv+dy+wdz is an exact differential, 
that Poisson has mentioned a case where this is not true, 
although the motion is small. But the theorem is only true, 
for the case of small motions, when the initial velocities wp, v, 
w, are such that uw) dx + dy +w)dz is an exact differential 
neglecting squares of small quantities. For the equation 
duis dv 
dtdy dtdz 
caf soni t 2% and the same applies to the other two 
dz dy dx 
corresponding equations. This exception includes the case 
mentioned by Poisson. 
; : : ; du 
gives, on Integrating with respect to ¢, dy 
XI. Proceedings of Learned Societies. 
GEOLOGICAL SOCIETY. 
[Continued from vol. xxi. p. 561.] 
February 23, MEMOIR was read, entitled, ‘‘ Report on the 
1842. Missourium now exhibiting at the Egyptian Hall, 
with an inquiry into the claims of the Tetracaulodon to generic 
distinction,” by Richard Owen, Esq., F.G.S., &c. 
The author commences with some remarks on the manner in which 
the Missourium is set up, and after pointing out certain mistakes, as 
well as the readiness with which Mr. Koch, the proprietor, corrected 
an error respecting the first pair of ribs, he states that the necessary 
reform in the juxtaposition of other parts of the skeleton could be 
effected only at a great expense. 
Mr. Owen then proceeds to consider the species of animal to 
which the skeleton is to be referred. It was, he says, a mammi- 
ferous animal, and while the anterior extremities disprove the ex- 
istence of clavicles, they establish that the fossil belonged to the 
Ungulata. The enormous tusks of the upper jaw further show that 
it was a member of the proboscidean group of Pachyderms, and that 
the molar teeth prove it to be identical with the Tetracaulodon or 
Mastodon giganteum. With respect to the horizontal position of the 
tusks in the skeleton exhibited at the Egyptian Hall, Mr. Owen 
states, that it may have arisen from compression, the tusk of the Mas- 
