280 
INl^ERIOR OOLITE AMMONITES. 
Gatull. Bumortieri. Two views of an immature specimen — a poorly-preserved 
cast — are shown in figs. 10 and 11 of PI. XXXIX. 
Catulloceras aratum/ 8. Buchnan. Plate XXXIX, figs. 1 — 5. 
Discoidal, compressed, carinate. Whorls quadrangular in form, ornamented 
with direct, ventrallj-inclined ribs, which are slightly reclined^ laterally. Ventral 
area fairly broad and defined, ornamented with a well-marked carina and a furrow 
on each side thereof. Inner margin slightly defined, smooth, sloping. Umbilicus 
regularly graduated. Inclusion about one-third. Suture-line with rather broad 
superior lateral lobe slightly longer than siphoual; inferior lateral lobe and 
auxiliary slightly retracted. 
The two specimens figured do not quite agree one with another; for the 
smaller one is certainly thinner in j)roportion, its ribs are less reclined laterally, 
and its whorls are more compressed ventrally. 
The most interesting point about these specimens is their ventral furrows, 
which apparently indicate parallelism of development with Arnioceras, Asteroceras, 
&c., of the Arietidae. In each phylum independently the ventral area developed 
as follows : Uncarinate, carinate, carinate and sulcate. 
To say that these furrows are due to atavism is certainly incorrect, just as it 
is to argue that they indicate any actual genealogic connection between this 
species and the Arietidse. They are really the result of what may be described as 
a dormant or latent hereditary force. The two families Arietidm and Polymorpkidse 
being descended from some common ancestor may be supposed to have derived 
from it the same hereditary tendencies, including the power to develop in certain 
directions. In the Arietidse the power of development manifests itself very 
quickly; and among other characters the sulcate-carinate ventral area is evols'ed, 
and carried to great perfection. On the other hand, the PoltjmorpMdse lasted for 
a very long time without much change ; then they began to develop, and produced 
features similar to those of the Arietidse, owing to dormant hereditary force, ^ only 
^ Described in the explanation of PI. XXXIX, figs. 1 — 5, as Dumortieria arata. The reader is 
requested to alter this to Catulloceras aratum. 
2 Ribs which fall behind a radial line drawn from the centre to the circumference through the 
inner end of the rib. 
3 Hyatt would, I presume, describe these furrows of Catull. aratum, in connection with those of 
the Arietidse, as homoplastic and not homogeuous. It may be observed that homoplastic features may 
be contemporaneous or not. Here, of course, they are not. I take this opportunity to refer the reader 
to Prof. Hyatt's splendid work on the Genesis of the Arietidse (' Smithsonian Coutrib. to Knowledge,' 
G73, 1889). 
