Vol. 62.] BRACHIOPOD HOMCE03IOKPHY. 451 



successive horizons : A. Lamberti, A. cordatus, A. altemans, typify 

 the three successive series of forms. Till recently all these, with 

 some other homceomorphs which lacked the knotted keel, such as 

 A. oxynotus, A. fissilobatus, A. Truellii, were placed in one genus, 

 Amaltheus. Now it is recognized that these various forms belong 

 to different genetic series, and that the Pliensbachian and Oxfordian 

 groups belong to widely-different families. The genetic series 

 have received different generic names : margaritatus is Amaltheus ; 

 spinatus, Paltopleuroceras ; Lamberti, Quenstedtoceras ; cordatus, Car- 

 dioceras ; alternans, Amoeboceras ; while of the homoeomorphous 

 forms, oxynotus is Oxynotoceras, a degenerate Arietan ; fissilobatus is 

 a phylogerontic Sonninia ; and Truellii is Stric/oceras, of the family 

 Oppelidae. And further, in each of these various genera many 

 species are recognized, although once it was freely argued that some 

 of these different genera were but varieties of a single species. 



The separation into genera of the similar forms of diphyoid Tere- 

 bratuloids is quite in accordance with this precedent. What was 

 the generic grouping of the knotted-keeled Ammonites 20 or 30 

 years ago is the position of the diphyoids to-day. What is the 

 generic grouping of those Ammonites to-day may be expected to be 

 parallel to the grouping of the diphyoids in the future. So that 

 the present generic grouping of those Ammonites may be regarded 

 as the prenguration of what the diphyoid grouping will be. 



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