* 
REVIEWS. 233 
n Amblystoma, and presents itself to the geologist when compared to 
je lower Clymenise in the same way, the only difference being that in 
this case the characteristics of a different order of animals are produced 
by the acceleration of the growth, instead of a distinct family and genus 
merely. 
Other instances are brought forward in the memoir referred to above 
de whic 
demonstrate with equal clearness the agency of the law acceleration 
in the production of varieties and even of individual differences. 
Thus one of the best known species of the Lower Lias, dead os (Am- 
monites) obtusum, is divisible into several varieties. For the sake, how- 
ever, of reducing it as much as possible we will eliminate all of these but 
three, and consider only the English specimens from one locality, Lyme 
Regis. These have three distinct variations of form. The first has the 
exceedingly shallow channels, while the pile (coste) are prominent and 
ither end. The channels appear on the last quarter of 
the third, and almost immediately attain their ultimate adult depth and 
aspect on the fourth volution; the second ame aiite in 
the larger number of individuals, but accelerates them by a t 
depth of the channels and the height o keel after.the Nin: volution, 
producing thereby adults with deeper channels and more prominent keels. 
There are different degrees of this acceleration in different individuals, 
some having shallower channels than others. 
The third variety attains the adult characteristics of the most ciem 
members of the second variety on the fourth whorl, and on the fifth, 
flattens the sides. The first and second varieties have gibbous or dime 
sini but the third is a transitional variety, approximating to Asteroceras 
stellare. The accelerations show themselves also in the development of 
the pile; the s rm 
these lateral projections at an earlier age than the first, and the latter 
forms pas same parts at an earlier age than in the first variety. 
This whole progress in the form and characteristics of parts takes place 
may and probably will be made to this view, that the third is really a va- 
riety of Asteroceras stellare, and does not belong to Asteroceras obtusum 
at all. This alternative would be even more favorable to the theory 
here advanced neis that given above. The difference is less in all re- 
pect wee 
Asteroceras obtusum, than between the former and Asteroceras stellare. 
Therefore any estimation of the value of their characteristics which would 
join the third variety to the latter species must also include the former 
AMER. NATURALIST, VOL. IV. - 80 
