80 GENESIS OF THE ARIETID. 
the group originated. That these reversions are the remnants of the earliest ac- 
quired structures and physiological powers seems perfectly plain, in view of the 
well known case of the return of childish structural peculiarities and memories in 
man after his adult peculiarities and powers have been exhausted. 
The peculiarities of series which, like Oxynoticeras, presented certain highly 
progressive or novel characters in combination with retrogressive characters, 
have been sufficiently described in these pages. It only remains to add, that 
such types are not uncommon in the different families of the Ammonoids and 
Nautiloids, and therefore they must not be considered as unique.’ 
DIFFERENTIAL CHARACTERISTICS. 
The differential characteristics have already received a considerable share of 
attention, but it still remains to review them in each series. The diagnosis 
of each genus is necessarily deceptive, in so far as it gives false views of the 
invariability of the differentials. 
The psiloceran series presented an altogether peculiar helmet-shaped whorl, 
with more decided congeners in the Trias than in the Lias. The involution 
increased in successive species, and in correlation with this tendency the complhi- 
cation of the sutures also became greater, 
The marked differentials of Weehneroceras, which are transitional from the 
plicated forms of Psiloceras to the series of Schlotheimia, the retention of 
the psiloceran form and sutures, the geniculess pile, and the nascent channel 
on the abdomen are so obvious, that they need only be mentioned and atten- 
tion again be drawn to the very remarkable fact, that, as in Psiloceras, this 
series departed from the discoidal radical, and exhibited increase of involution 
in successive species. 
Starting from Psil. planorbe, var. plicatum, as the radical discoidal progenitor of 
the remainder of the Plicatus Stock of the Arietida, we find that the compressed 
helmet-shaped whorl was exchanged in Cal. Johnston’ for a more gibbous rounded 
whorl, but the discoidal character of the shell was maintained, and the pile did 
not have genicule or tubercles except in the highest species. There was also a 
tendency in Cul. Johnston’ towards a complication of the margins of the sutures 
through the deepening of the lobes and saddles, which was especially noticeable 
in Cul. nodolianun. This increase of complication took place especially in the 
marginal lobes, and there is a backward trend of the auxiliary lobes and saddles, 
which causes a close likeness between the tendency of the progression in this 
genus and that of the involute forms of Psiloceras. In Caloceras, however, it 
1 We can mention as similar cases the following: Subclymenia with its ventral lobe and ventral siphon, 
a true Nautiloid of the Trigonoceratide; Pteronautilus among the Gonioceratide with its winged aperture; 
Centroceras among the Hercoceratide with a deep V-shaped ventral lobe. Among Ammonoids there are 
the genera Pinnacites and Celmceras with remarkable sutures among Nautilinide ; the Gonioclymenide 
with ventral lobes instead of continuous saddles in the Clymenine; Beloceras with its extraordinary sutures, 
and Medlicottia with its remarkable ventral lobe and first pair of saddles among the Prolecanitide ; and a 
host of others. 
