Paleontology. 201 
Planorbis. The following is an authoritative résamé 
of the facts. 
As the deposits seem to have been continuous for ages, and 
the fossil shells very abundant, this seemed to be an excellent 
opportunity to test the theory of derivation. With this end in 
view, they have been madc the subject of exhaustive study by 
Hilgendorf in 1866, and by Hyatt in 1880. In passing from the 
lowest to the highest strata the species change greatly and many 
times, the extreme forms being so different that, were it not for 
the intermediate forms, they would be called not only different 
species, but different genera. And yet the gradations are so in- 
sensible that the whole series is nothing less than a demon- 
stration, in this case at least, of origin of species by derivation 
with modifications. The accompanying plate of successive forms 
(Fig. 89), which we take from Prof. Hyatt’s admirable memoir, 
will show this better than any mere verbal expianation. It will 
be observed that, commencing with four slight varieties—pro- 
bably sexually isolated varieties—of one species, each series 
shows a gradual transformation as we go upward in the strata— 
i.e. onward in time. Series I branches into three sub-series, in 
two of which the change of form is extreme. Series IV is re- 
markable for great increase in size as well as change in form. 
In the plate we give only selected stages, but in the fuller plates 
of the memoir, and still more in the shells themselves, the sub- 
tilest gradations are found}. 
Here is another and more recently observed case of 
transmutation in the case of mollusks. 
The recent species, Strombus accipitrinus, still in- 
habits the coasts of Florida. Its extinct prototype, 
S. Leidy, was discovered a few years ago by Prof. 
Heilprin in the Pliocene formations of the interior 
of Florida. The peculiar shape of the wing, and 
tuberculation of the whorl, are thus proved to have 
grown out of a previously more conical form of 
shell. 
1 Le Conte, Zoe. czt., pp. 230-7. 
