gO GENESIS OF THE ARIETID.E. 



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. 1 



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 compli- 

 cation of the sutures also became greater. 



The marked differentials of Wsehneroceras, which are transitional from the 

 plicated forms of Psiloceras to the series of Schlotheimia, the retention of 

 the psiloceran form and sutures, the geniculess pilae, 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. plicatwn, as the radical discoidal progenitor of 

 the remainder of the Plicatus Stock of the Arietidae, we find that the compressed 

 helmet-shaped whorl was exchanged in Gal. Johnstoniiox a more gibbous rounded 

 whorl, but the discoidal character of the shell was maintained, and the pilae did 

 not have genicular or tubercles except in the highest species. There was also a 

 tendency in Gal. Johnstoni towards a complication cf the margins of the sutures 

 through the deepening of the lobes and saddles, which was especially noticeable 

 in Gal. nodoiianum. 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 Trigonoceratidse ; Pteronautilus among the Gonioceratid.-e with its winged aperture; 

 iceras among the Hercoceratidie with a deep V-shaped ventral lobe. Among Ammonoids there are 

 the genera Pinnacites and Celseceras with remarkable sutures among Nautilinidse ; the Gonioclymenidaj 

 with ventral lobes instead of continuous saddles in the Clymeninse; Beloceras with its extraordinary sutures, 

 and Medlicottia with its remarkable ventral lobe and first pair of saddles among the Prolecanitidje ; and a 

 host of others. 



