1890.] Geology and Paleontology. 557 
General Notes. 
GEOLOGY AND PALEONTOLOGY. 
The Genesis of the Arietidae,—This important memoir, by Prof. 
A. Hyatt, is the result of an effort to find a real demonstration of the the- 
ory of evolution. The methods of analysis used show the origin and rise 
of the ten series of species from one variety of one species, Pszloceras 
planorbe, the Ammonites planorbis Sow., and Ammonites psilonotus of 
Quenstedt. There are two varieties of this species, one smooth and one 
plicated. The smooth variety is the oldest in point of time of occurrence, 
and the development of the plicated variety, as well as its more recent 
station in time, show that it is a descendant of the smooth variety. 
The smooth variety is the ancestor of a series in which the forms be- 
‘come more involute and have more complicated sutures, but are smooth 
and have no keels, so that they may be accurately said to belong to the 
same genus, Psiloceras, as their smooth ancestor. This is pictured in 
Summary Plate XIV. as the central stock. On the right of this six 
series or genera are arranged, showing how these sprang, either directly 
or indirectly, from the same smooth variety of Psz/. planorbe. On the 
left of the central stock or genus Psiloceras, four series are represented 
so as to show how these arose from the plicated variety of Psz/. planorbe. 
Each series is in each case described as a distinct genus,—in fact the 
idea of the genus is founded upon its separability as a series of species 
branching off from the main stock or radical form. The last allusion to 
a cycle is due to the fact as shown in the Summary Plate XIV. that in 
each of the series there is a similar succession of forms. The forms 
from which each series arose were discoidal or open shells, with rounded 
whorls showing the internal coils plainly. ‘As each series of forms was 
evolved from central stock or radical form in diverging lines like 
the spokes of a fan, each produced with curious iteration quad- 
ragonal whorls with keels and channels, or one or the other of these, 
along the periphery, and became covered with ribs on the sides. After 
this the successive species in each series became more and more invo- 
lute in eight out of the eleven series; they lost their keels and 
channels, and their whorls became compressed, the abdomens at the 
same time tending to become acute. Thus the series of species, although 
