317 
forms are not confined to special localities, as in the Jura, but are 
found in all faunas so far as known. 
The facts show that some general physical cause acted simulta- 
neously, or nearly so, over the whole of the known area of the 
world during the Cretaceous period, and produced precisely similar 
effects upon the whole type as had here and there been noticeable 
only within limited localities and upon single species or small num- 
bers of species during the previous periods. This general cause, 
whatever it may have been, affected the type so as to cause the suc- 
cessive generations of the larger part of the shells to become dis- 
torted, smaller and more cylindrical in their whorls, smoother and to 
lose their impressed zones and their complicated foliated sutures. 
In extreme cases they became again, with the exception of the 
earliest stages which are usually broken off and lost, perfectly straight 
cones, like the orthoceratitic radicals. So much alike are they, that 
it is quite common for those who are not students of this group to 
mistake the degraded Baculites for the radical Orthoceras. This 
decrease in size, increasing smoothness, and uncoiling, is precisely 
parallel with the similar transformations taking place during old age 
in the normal involute shells of the Jura, which, when old enough, 
also depart from the spiral, or tend to straighten out, and always 
lose their ornaments, decrease in size, and so on. 
The universal action of the surroundings, as we now know them, 
is certainly not exclusively favorable to the continuance of life, and 
may be wholly more or less unfavorable. It certainly perpetually 
excites the animal to new and more powerful exertions, and, like 
perpetual friction, wears out its structures by the efforts which it 
obliges it to make for the support of the structures in doing work. 
At first this leads to development, the supply being greater than the 
demand ; but sooner or later, and with unvarying certainty, the de- 
mand exceeds the powers of supply, and old age sets in, either pre- 
maturely, or at the termination of the usual developmental periods. 
The remarkable and at present unique example of the Ammonoidea 
places us in a position where we can see the same process taking 
place in the whole of a large group, with attendant phenomena 
similar in every respect to those which we have observed in indi- 
vidual shells of the same order. 
In numbers of species and genera, and in the complication of the 
internal structures and the production of the external ornaments on 
PROC. AMER. PHILOS. SOC. XXXII. 143.2 Vv. PRINTED MAY 31, 1894. 
