PARALLELISM. 189 



my ' Genesis of the Arietidae ' 1 and others have been 

 given by Buckman and Wiirtenburger. 



"Some of the most remarkable occur in the least 

 expected quarters. As usual, when one has a true law, 

 it leads him into conclusions that are, perhaps, more 

 surprising to himself than to his readers, or to any 

 subsequent investigator. This was certainly my own 

 case in being led to recognise the perfect examples of 

 parallelisms in retrogressive series. Quenstedt and 

 all students since his time agree that the so-called 

 genera Crioceras, Hamites, Ancyloceras, Baculites, 

 forms that are successively more and more uncoiled 

 until in Baculites they are absolutely straight cones, 

 were derived from normal, close-coiled, involute-nau- 

 tilian shells of the Ammonitinag. Their young have 

 been repeatedly shown to be close-coiled and they 

 grade into the normal progressive shells by all of their 

 adult characters. 



"The ultimate fact in this demonstration has been 

 added by Dr. Amos Brown in the discovery of a close 

 coiled nepionic stage in the straight Baculites of the 

 Cretaceous, the only form whose development had not 

 been ascertained and whose exact "relations had not 

 been determined. 



"It is now admitted by all students of Ammonitinse 

 that these retrogressive groups are not true genera; 

 but that as first demonstrated by Quenstedt, Baculites, 

 Crioceras, etc., are retrogressive stages in the evolu- 

 tion of distinct genetic series and that they do not ex- 

 ist as natural groups of species. In other words, dif- 

 ferent genetic series of the Ammonitinse die out by 

 passing through a series of modifications which are 

 parallel and which are just the reverse of the parallel 



1 Smithson, Contrib., 673, p. 41 et seq. 



