654 
BIOLOGY: L. /. HENDERSON 
of quadratojugals^ dermosupraoccipitals and tabulars has been defi- 
nitely determined, and the supratemporals almost as definitely, thus 
giving to the genus nearly every skull element found in the most prim- 
itive cotylosaurs. The mandibles may be a little less slender and there 
may be fewer teeth but one can say with assurance that were an isolated 
skull of Sphenacodon found in Texas it would be referred unhesitat- 
ingly to Dimetrodon. And this similarity extends to other parts of the 
skeleton, with the exception of the spines of the vertebrae. 
Many and various have been the conjectures offered to explain the 
extraordinary spines in Dimetrodon. In my opinion nothing better 
has been suggested than that of Professor Case, namely, that they repre- 
sent an ornamental or senile character of the race, of no profound im- 
portance in the life economy of the animals. Certainly, as I have pre- 
viously observed, if the spines of Dimetrodon had been of important use 
to their possessors they must have produced correlated differences in 
other parts of the skeleton. The differences in the spines have no more 
than a generic value. 
There are doubtless several species of Sphenacodon in the New Mex- 
ican beds, but the known remains from the Baldwin and Miller bone- 
beds seem all to belong to one species. From different horizons on the 
Puerco, however, large specimens are known, now preserved in the 
Yale Museum. The present specimen, which doubtless belongs to 
S. ferox Marsh, was nearly five feet long as figured. The Yale speci- 
mens indicate a form one-half larger, or about seven and a half feet in 
length, or of about the size of most specimens referred to Dimetrodon 
incisivus. Sphenacodon is definitely known only from the valley of the 
Puerco and its tributaries in New Mexico; not a fragment of it has been 
found in the El Cobre deposits scarcely a score of miles away. How- 
ever, there are specimens in the Chicago collection from Texas that 
suggest very strongly its occurrence there, but definite proof is 
lacking. 
ON VOLUME IN BIOLOGY 
By Lawrence J. Henderson 
CHEMICAL LABORATORY OF HARVARD COLLEGE 
Receivad by the Academy. October 28, 1 9 1 6 
In one respect this subject is time-honored. Herbert Spencer and 
many others since his day have recognized the biological importance of 
the principle of similitude. Bulk increases as the cube of length; 
surface as the square. Hence the limitation of the size of cells, the 
