Hyatt.] 30 [October 5 : 



row of spines, to complicate the septa and increase the size, the law 

 of acceleration is brought into full play, and overcoming the tend- 

 ency of the species to be arrested in development both of size and 

 characteristics, counteracts this tendency and reproduces the usual or 

 natural succession of forms and characteristics. 



This may be substantiated in any series of Ammonites. By com- 

 paring the lower forms with the higher of the same series it will be 

 found that in most instances, when the series is complete, the spe- 

 cies, as in Androgynoceras and Liparoceras, increase the extent of 

 the involution and the number of lobes. This is precisely what 

 occurs in the Arietidse, which are even more successful in suppress- 

 ing the reversionary planicostan tendencies than the Microceran 

 series. 



In this family the higher forms, Asteroceras stellare, Asteroceras ac- 

 celeratum 1 and others, are much more involuted than any of the lower 

 forms, and this is still more strongly expressed in their descendants, 

 the Amaltheoida? and Hildoceratidaa of the middle and upper Lias. 



It may be objected that Microceras biferum is a young form of 

 which we do not yet know the adult. Its size, the limited number 

 of the whorls and the likeness of the septa, in the full grown speci- 

 men, to the young of Deroceras Dudressieri and Microderocer as Birch ii 

 might be considered as proof of this supposition. The development 

 is just intermediate between that of laticosta and Bircliii', any larger 

 forms could therefore only intensify this relation. 



Besides the negative evidence, however, that no large specimens 

 have ever been found, there is something positive. 



The possession of prominent tubercles makes it probable that quite 

 an advanced stage of life is reached, since at a corresponding age in 

 laticosta no spines are yet developed. 



Similar doubts with regard to the size of planicosta and confusum 

 in the Deroceran series are answered with more difficulty. The grad- 

 ual decrease in size which the series makes from Microderoceras Bircliii 

 through D. Dudressieri, D. zipldus and D. planicosta to D. confusum, 

 in all the dimensions of its whorls, when the full sized shells are 

 considered, and the fact that these species, especially D. planicosta, 

 have been very extensively collected, appear to make it probable 

 that we now know the shells as they occurred in the localities and 

 strata in which they are found. That they may be dwarfed speci- 



1 New species, which has the abdomen like Aster obtusus, but is more involute 

 than any other species of Arietes. 



