No. 467.] MOMENTUM IN VARIATION. 841 
symmetry. These, with the symmetry of sessile crinoids, sea 
anemones, and floating jelly-fishes offer illustrations of the carry- 
ing of a feature far beyond utility, toward perfect symmetry. 
The molluscan Hippurites starts as a form which thickens its 
shells, one of them especially; apparently asa protective measure. 
But when the shell has reached a thickness of an inch it can 
hardly be considered useful to increase it further. However, 
these forms go on increasing in thickness to in some cases over 
a foot, using up the vitality which would otherwise be available 
for reproduction. Momentum seems to be the only reasonable 
explanation in such cases, and there are several (Coralliopside, 
Gryphea, Caprinidze, etc.). 
Among Cephalopoda the complication of the edge of the sep- 
tum of various Ammonites offers another case. A sinuous mar- 
(a B1 
| | ^ 
s v2 i h Z : d : / 
R Pu Pa i ia; Des- 
Fic. 2. — Series showing suture lines of Ammonites. @, Goniatites; 4, Medliocottia: c, 
moceras. 
gin is doubtless a better brace against the pressure of the outside 
sea water, and such an irregular line as that of Goniatites or even 
Medliocottia (see Fig. 2) would be interpreted as useful. How- 
ever, when it is carried to the complication found in Desmoceras 
or Pinacoceras, only momentum seems to explain the condition. 
Turning to the vertebrates where the balance required for 
survival is even more delicate, the tooth of the Labyrinthodonts 
