June, 1891 . 
ANIMAL PEDIGREES. 
129 
species are known to us, of many of which large numbers of 
specimens can be obtained, in excellent preservation. The 
group consequently is a very suitable one to study from our 
present standpoint; and the enquiry gains additional interest 
from the fact that Ammonites are an entirely extinct group 
of animals, no single species having survived the cretaceous 
period, so that our only chance of learning anything about 
their embryology is to study the fossil shells themselves. 
Wiirtenberger, who has made a special study of the Jurassic 
Ammonites, has shown that there is the same correspondence 
between historic and embryonic development that obtains 
among living animals. In the middle Jurassic deposits, for 
instance, the older Ammonites are flattened and disc-like, 
with numerous ribs ; in later forms the shell bears rows of 
tubercles near the outer side of the spiral, and later still a 
second inner row of tubercles as well, while the ribs gradu¬ 
ally become less conspicuous, and ultimately disappear. In 
forms from more recent deposits the outer row of tubercles 
disappears, and then the inner row, the shell becoming 
smooth, swollen, and almost spherical. 
On taking one of these smooth, spherical shells, such as 
Aspidoceras cyclotum, and breaking away the outer turns of 
the spiral so as to expose the more central and older turns, 
Wiirtenberger found first an inner and then an outer row of 
tubercles appearing, which nearer the centre disappeared, 
and in the oldest part of the shell were replaced by the ribs 
characteristic of the earlier, and presumably ancestral forms. 
Results such as these open up to us a new field of enquiry, 
which, if energetically worked, must yield results of great 
interest and importance. 
In order to understand fossils aright, and to derive from 
them the full amount of information they are capable of 
yielding us, it is necessary «tliat we should have thorough 
knowledge of the development of their living descendants; 
and more especially that we should be fully acquainted with 
the several stages of formation of the shells or other hard 
parts of the recent forms, which, in their fossil representa¬ 
tives, are, with rare exceptions, the only parts sufficiently 
well preserved to give trustworthy evidence. 
Embryologists have too often confined themselves to the 
earlier stages of development, and have unduly neglected the 
later stages, and more especially the later stages of the 
skeletal structures. By so doing they have failed to afford 
to palaeontologists the aid which they are peculiarly qualified 
to give, and which to the palaeontologist would be of the 
utmost value. Fortunately, the mistake is now recognised, 
and serious efforts are being made to remove the reproach. 
(To be continued.) 
