128 GENESIS OF THE ARIETIDA. 
but in the adults of involute species the whorl is necessarily more compressed. 
The compressed stage occurs very early in the most involute species, the flatten- 
ing of the sides and the depressed abdomen being omitted. A distinct median 
channel is formed on the abdomen in all species except some varieties of Schiot. 
catenata. The pile cross the abdomen in the earlier nealogic stages, but this 
peculiarity is rarely retained in adults except in catenata. The channel is formed 
by the suppression of the pile along the median zone of the abdomen, and is 
sometimes, especially in the young, supplemented by the bending inwards of the 
shell, This channel is converted into a smooth zone in old age by the degen- 
eration and disappearance of the geniculw, and the tendency of the abdomen to 
become narrower elevates this zone and makes the whorl subacute. Involution 
so far as known does not decrease in old age, and while it is easy to separate the 
senile stages of involute species from the senile stages of any species of Waeh- 
neroceras, it is not practicable to distinguish those of the discoidal species until 
the ephebolic stages are studied. The specimens figured by Quenstedt* show 
that, in extremely aged specimens, the abdomen becomes in some cases 
rounded, and it is instructive to compare Fig. 10m, Plate IIL, with the aged 
Psiloceras, Fig. 1m, in order to see how complete the reversion occasioned by 
senility may sometimes become. 
The sutures are not distinguishable from those of Weehneroceras. They are 
perhaps less like. those of Caloceras than those of that genus. The superior 
lateral lobes also are usually not so long and narrow, nor the superior lateral 
saddles so large and deep, nor the auxiliaries so much inclined posteriorly. 
The sutures are similar, both during the nealogic and senile stages, to those 
of Weehneroceras, and the differences, if any can be detected, occur only in 
the adult stages. 
Wihner’s plates? are so complete, that one can study the history of the devel- 
opment of each form, and the relations of the species in their nealogic stages. 
The young of the more involute species, like Schdot. ventricosa and marmorea, are 
similar to the later nealogic stages of less modified and more discoidal forms, like 
Schlot. donar, and are also similar to the adult stages of still more modified species, 
like Sehiot. angulata. These facts confirm the opinions we have advanced in the 
description above, and in other parts of this memoir.’ 
1 Amm. d. Schwab. Jura, pl. iii. and iv. 
2 Mojsis. eb Neum., Beitr., IV., 1886. 
2 We have several times referred in this memoir to extraordinary parallelisms. But we know of none 
more remarkable than those figured by Canavari in his ‘‘ Fauna der Unteren Lias von Spezia.” We refer 
to the genus Ectocentrites of Wihner, in which the young as described by Canavari are similar to Lytoceras, 
while the later nealogic and adult stages have the pile: and abdominal channel of Schlotheimia. 
