GENESIS OF THE AKIKTI D.K. 



1\ PRODI CTION. 



OrIQIH and Cfl \i:\i 1 1 i:isi [( - HOKDERS. 



r*HE >n of forma among the Bilurian members of tin- genua Mimoo 



A indicates that I eran shells occurred among Ammonoidea, differing 



•In' -imilar forme among Nautiloida only in tin- possession of a globular pro- 

 small ventral lobe. In Borne Bilurian and devonian Anarcestes these 

 permanent adull Btagea are repeat *1 in tin- development of the young Those in 

 aii- trnh cyrtocerun, or open curvea at first : ami in others, 

 a- in a variety of. I .iil.nl by Barrande, they arc Btraight 



'I In- next stage ol growth i> a loose-coiled or gyroceran form, like the adull 

 in only !i«- accounted for a- hereditary tendencies 

 >wtb in a type which is being rapidly changed from a primitive ancestral 

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



describes ami figures a specimen of Bactrites with a protoconch 

 similar to the very peculiar ovoid protoconch of Mim. compression. Se i|ii<>tis 



i ive him this s] imen, as authority tor the view that Bactrites 



th Mimocei B ilitea is with tin- normal Ammonoids of the 



1 This idea was first published by Qucnstedt in his "Die Cephalo- 



' L Bactril of the Devonian may Kr a de- 



i form '>: M .-. Inn in that case tin- latter i- also -a degraded form of 



Anarcestes, or transitional between it and Bactrit* I establish this proposi- 

 tion, fo • M im>1 Anarcestes should be produced in which uncoiling 

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



Such degraded forms arc- « i in the Jura and Cretaceous, and enable the 



B tlitea with the normal coiled Ammonoida of the same 

 formations. Whether th or not, the straight Bactrites-like young of 



some forms of Anarceatea, the in young of others of the Goniatitinas, 



anil t ; adults and yoimg of Mimoceraa, indicate the derivation of 



titinso to have 1 n from sAurian I shells similar to Bactriti 



not din hi- itself 



( : . 



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