1870.] 29 . [Hyatt. 



the form and septa. One is a greater degree of involution, the outer 

 whorls as they grow, beginning to spread laterally over the sides of 

 the inner whorls, and the septa keeping pace with the increased 

 breadth of the sides, adding to the number of the auxiliary or inner 

 lobes and cells. This higher degree of complication is carried to its 

 greatest development in Liparoceras ; which, however, in its highest 

 species, Liparoceras Bechei, returns wholly to the mode of growth 

 originally observed in Microderoceras Bircliii. It proceeds directly 

 from the young, smooth, round-abdomened stage, to produce the 

 double row of tubercles, without the interpolation of the planicostan 

 characteristics. It may be possible that the planicostan stage occurs 

 in some individuals, but this would only complete the parallel with 

 Bircliii which sometimes faintly expresses this reversionary feature. 



The conclusion with reference to this series appears to be, that 

 its members are also at first degraded descendants of Bircliii, 

 but instead of steadily decreasing in size and ceasing to exist, they 

 first decrease and then speedily increase in size again, adding new 

 elements of complication to the mode of involution, and increasing 

 the number of the lobes and cells. All my attempts to trace a 

 direct connection with those members of the Psiloceratidaa and 

 Arietidse, which approximate to these series, have signally failed. 

 The planicostan abdomen and the similar septa and forms which 

 are found in the adults of Psiloceras, Caloceras, 1 and Ophioceras, 

 and in Microceras and Deroceras can be viewed merely as reversions, 

 indicating, as in the different breeds of pigeons, only a common 

 ancestry. 



It should be observed also, that where reversion is apparently 

 piled upon reversion, as 'for instance, in the return of the Birchean 

 characteristics in Androgynocer as, after an interval caused by the pre- 

 potent development of the planicostan abdomen, and an interval 

 of time also, that this is not a reversion at all. It is, in fact, the re- 

 sumption of a normal tendency beneficial to the race, which for a 

 time has been entirely suppressed by the prepotent influence of a 

 true reversionary feature. 



This can be doubly proved. In the Deroceran series, where no 

 tendency to increased complication or size is observable, the race 

 becomes enfeebled and dies out almost immediately. In the Microce- 

 ran series, where a constant effort is observable to retain the double 



1 A new genus, of which Caloceras torus and tortilis are types. 



