GENESIS OF THE ARIETID^E. 



INTRODUCTION. 



Origin and Characteristics of Suborders. 



HPHE succession of forms among the silurian members of the genus Mimoceras 

 •*• indicates that true gyroceran shells occurred among Ammonoidea, differing 

 from the similar forms among Nautiloids only in the possession of a globular pro- 

 toconch and a small ventral lobe. In some silurian and devonian Anarcestes these 

 permanent adult stages are repeated in the development of the young. Those in 

 Mimoceras compression are truly cyrtocenin, or open curves at first ; and in others, 

 as in a variety of Anarcestes fecundus described by Barrande, they are straight. 



The next stage of growth is a loose-coiled or gyroceran form, like the adult 

 of Mimoceras. These stages can only be accounted for as hereditary tendencies 

 of growth in a type which is being rapidly changed from a primitive ancestral 

 straight form with simple sutures into a close-coiled nautilian shell. 1 



Branco 2 describes and figures a specimen of Bactrites with a protoconch 

 similar to the very peculiar ovoid protoconch of Mini, compression. He quotes 

 Beyrich, who gave him this specimen, as authority for the view that Bactrites 

 is connected with Mimoceras as Baculites is with the normal Ammonoids of the 

 Cretaceous. This idea was first published by Quenstedt in his " Die Cephalo- 

 poden," and it is quite possible that Bactrites of the Devonian may be a de- 

 graded form of Mimoceras, but in that case the latter is also a degraded form of 

 Anarcestes, or transitional between it and Bactrites. To establish this proposi- 

 tion, forms of Mimoceras and Anarcestes should be produced in which uncoiling 

 occurred in adults after a close-coiled stage of growth had been passed through. 

 Such degraded forms are common in the Jura and Cretaceous, and enable the 

 observer to connect Baculites with the normal coiled Ammonoids of the same 

 formations. Whether this be so or not, the straight Bactrites-like young of 

 some forms of Anarcestes, the gyroceran young of others of the Goniatitina?. 

 and the gyroceran adults and young of Mimoceras, indicate the derivation of 

 Goniatitina?. to have been from silurian straight shells similar to Bactrites, if 

 not directly from that genus itself. 



1 Genera Foss. Ceph., pp. 303, 304, 309, Proc. Bost. Soc. Nat. Hist., XXII., 1883. 



2 Zeitscu. Deutsch. Geol. GeselL, XXXVII. p. 1. 



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