Hyatt.] 32 [October 5, 



It will be noticed that these reversionary species 1 all descend from 

 one, to which they may be traced by all the evidences within the 

 scope of observation, and that this single ancestor has occasionally 

 in its own development, characteristics which do not occur in its own 

 series in any of the faunas of the lower Lias below its own level, and 

 between it and the Trias. 



The objection will naturally suggest itself, that perhaps Microdero- 

 ceras Birchii is a migratory species from India, or somewhere out of 

 Eastern Europe, and that in its native haunts we shall probably find 

 the missing links which connect it with the Trias, and farther find 

 that these have the same reversionary features in their growths. 

 But it must be remembered that the planicostan abdomen occurs in 

 some individuals only, a fact very strongly in favor of the supposition 

 that it is a reversion. Darwin's observations seem to establish the 

 fact, that reversions are transient characteristics, and peculiarities 

 directly inherited are, on the other hand, more or less constant, ap- 

 pearing in every individual of the species. Farther, the Arietes are 

 a group native to Eastern Europe, during the Lias, and they most 

 unquestionably revert just as the young of Microderoceras Birchii, 

 and in precisely the same transient manner, to the planicostan abdo- 

 men, — or rather, as it ought to be called, the Triassic abdomen, in 

 allusion to the age from which it is derived. 



LIPAROCERATID^. 



MICRODEROCERAS. 



Microderoceras Birchii. 



Amm. Birchii Sow., Min. Conch., vol. Ill, p. 121, pi. 267. 



This well known species has septa which are different from those 

 of the so-called Amm. brevispina of D'Orbigny. A perfect specimen 

 of the French brevispina possessed by the Museum is a much smaller 

 shell than the M. Birchii, having fewer whorls and entering upon 

 the old age period, whilst the typical Birchii is still in its prime. In 

 the young the tubercles and pilas of brevispina are just as prominent 

 during the younger stages of growth as in Birchii, but in the adult 

 the spines and pilas are less prominent, though the latter are more 

 closely set upon the sides of the whorls. The septa, according to 



nient. 



And I might add other species, which are not necessary to the present argu- 



