106 : REVIEWS AND BOOK NOTICES. 
pressus, and we see at once that the development of the same parts 
is very ‘much quickened or accelerated in the typical Ammonite. 
hat this acceleration of development is due to the prepotency 
of the same progressive tendency as the closer and closer coiling, 
and final involution of the ovisac, by the first whorl, can hardly be 
doubted. Thus, not only in the whole series of Nautiloids are the 
forms more or less completely coiled and finally enveloping, but 
in the young Ammonoids this process is repeated, but only as a 
reversionary tendency of individuals and species, or at most, per- 
? ` 
haps, by the group of Nautili. 
Our author finds that all the typical Ammonites may be resolved 
into natural series. As bearing upon the question of mimetic 
forms, we may refer to the observation that the above noticed series 
‘‘contain more or less representative or mimetic forms due to the 
resemblance occasioned by the amount of the involution or the 
characteristics which are usually correlative with the amount of 
involution.” He also indicates the effects upon the individual and 
the group to which it belongs of the changes due to old age, which 
have not been sufficiently taken into account by observers. 
by the encroachment of senile characteristics. These are observed 
th 
tance of senile characteristics is not claimed, but merely that 
the group are similar, and both due probably to the same cause, 
exhaustion of the powers of growth.” 
As an example of Cope’s law of “ retardation” in accounting 
for the origin of distinct forms, Prof. Hyatt cites the case of the 
spiral or Gasteropod genus Turrilites. 
_ “The young of several species of typical Ammonites often as- 
_ sume the spiral, although this is entirely suppressed at a later stage, 
_ and the succeeding whorls resume the normal mode of growth and 
_ revolve in the same plane. When, therefore, the normal mode of 
development is “retarded,” we find even in the adult this Turrilites- 
like condition of the young, which is as truly reversional as the 
retrogression of the individual in old age and the retrogression of | 
