128 GENESIS OF THE ARIETID^. 



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 Schlot. 

 catenata. The pilae cross the abdomen in the earlier nealogic stages, but this 

 peculiarity is rarely retained in adults except in caienata. The channel is formed 

 by the suppression of the pilee 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 genicular, 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 1 show 

 that, in extremely aged specimens, the abdomen becomes in some cases 

 rounded, and it is instructive to compare Fig. 10 m, Plate 111., with the aged 

 Psiloceras, Fig. 1 m, in order to see how complete the reversion occasioned by 

 senility ma}' sometimes become. 



The sutures are not distinguishable from those of Waehneroceras. 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 Waehneroceras, and the differences, if any can be detected, occur only in 

 the adult stages. 



Wahner's plates 2 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 Schlot. 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 Schlot. angulaia. These facts confirm the opinions we have advanced in the 

 description above, and in other parts of this memoir. 3 



1 Amm. d. Schwab. Jura, pi. iii. and iv. 



2 Mojsis. etNeuin., Beitr., IV., 1886. 



8 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 Wiihner, in which the young as described by Canavari are similar to Lytoceras, 

 while the later nealogic and adult stages have the pilae and abdominal channel of Schlotheimia. 



