REVIEWS AND BOOK NOTICES. 105 
“The range of form has been among the Nautiloids from the 
straight Orthoceratite through intermediate arcuate genera, to the 
pees Ronee Sage and finally the oon coiled Nautilus. 
Suc the case, if there is any trut e doctrine of evo- 
ERE we g AA expect to find some pjeses es the peculiarities 
of the parent Nautiloid stock in the earlier stages of development 
among the Ammonoids. And n as a direct and unavoidable 
corollary of the above, we ought to find this reference more dis- 
tinct in the young of the earlier ka of Ammonoids, the Goni- 
-atites of the Silurian, and less noticeable in Goniatites of the 
Devonian and Carbonifer erous, and, finally, almost obliterated, or at 
any rate, still less distinct in the typical Ammonites of the Jura. 
direction. The simple Nautiloid-like Goniatites of the Silurian 
may exhibit an Orthoceratitic or straight form, or be closely coiled 
in the young of different varieties of two distinct species. A 
species, therefore, on this horizon, ma have a range of variation 
r 
ment, either in time or in adult organization, of the Ammonoids 
from their supposed parent stock. There are evidently two ten- 
dencies at variance with each other: one strongly reversionary, 
appearing in the frequency with which the earlier Goniatites r 
the parent form in certain isolated instances in the young of va 
ties, and in the different species of the later Goniatites manifesting 
itself in the arcuate cone of the young of Goniatites compressu 
and others of the Nautilini, and in the closer, though non- pevelais; 
coiling of the young of other forms. Evidently this tendency i is 
losing its power to affect and modify the organization, or, in other 
wor ds, its prepotency. The other tendency, which is expressed in 
the closer coiling of the whorls, and — in their increasing 
involution, is decidedly progressive, increasing in power to the 
final and ultimate extinction of all reference te the ancestral type, 
except in the internal organization. Here, as will be shown, the 
siphon for a limited time remains central in the first whorl, and the 
first septum has a large entire abdominal cell, and simply concave 
lateral aati as in the Nautiloids. 
, however, of the first whorl of the Ammonoid is like 
Gonistites, the shell similar, and the second septum has the inva- 
tiable abdominal lobe, superior lateral cells, and lobes of the 
ita adult, Coulais. but not by any means of the simplest 
Goniati The simplest adult Goniatites have no proper lateral 
cells, > a broad lateral simple curves to the septa, as if the 
first septum of the Ammonite was modified or broken by a small 
abrupt lobe on the abdominal side. Contrast this with the devel- 
opment of the septa, and their gradual change in Goniatites com- 
