1909 



DEVELOPMENT 



XI 



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Among Nautiloids and Ammonoids, as a whole, the cyclical develop- 

 ment may be represented thus : — 



Table I — Coiling Cycle 



t y occlusal 



{Nautilus) 

 serpen tal 

 (Hercoceras) 

 ./^ 



gyral 

 {Gyroceras) 



I 

 curved 

 {Cyrioceras) 



•^ — straight 



(Orthoceras) (Baculites) 



These stages represent the complete cycle of development which 

 was accomplished from a straight cone through curving and incoiling 

 cones up to a cone with quite closed umbilicus, back again through 

 outcoiling and curving cones to a straight cone ; but there was always 

 power to reverse at any stage, without completing the cycle. Of these 

 terms, gyral signifies a loose volute ; serpenlal, a close volute, coiled like a 

 snake, with whorls touching or even lapping ; and occlusal, an embracing 

 or occluded volute — a cone so coiled upon itself that there is only a small 

 or even no umbilicus. 



The generic names appended stand merely as examples of the degrees 

 of coiling : they are not necessarily in genetic relationship. The arrows 

 refer to the cyclical development expressed by the adjectival terms, 

 and indicate the course of a complete cycle ; but this is rare : incom- 

 plete cycles or partial cycles, with subsidiary cycles, are the rule. Many 

 stocks end before completing the cycle. 



Hyatt expressed some of the forms of Cephalopod cones by combining 

 the word cone with generic names, making Orthoceracone, Nautilicone, etc. 

 In the etymological objection taken to such a term as Orthoceracone 

 for Orthoceratocone there is little ; but the ceras which seems to confine 

 the terms to Cephalopods can be omitted, with the advantage of making 

 them applicable to molluscs generally. The following may be 

 suggested : — 



Table II — Cone Stages 



u 



C 



Ophiocone 



^_ 



I 



Gyrocone 



Sphaerocone 



-y 



Serpenticone 



I 

 -V 



Criocone 



I 



o 



o 

 n 

 n 



r 

 G 



~ Cyrtocone Toxocone ^ 



•{ — Orthocone Baculicone «^— ^ 



Of these, Ophiocone ('"'/>t', a snake) covers Hyatt's discoidal, 

 that is, evolute Nautilicone ; SphcBrocone (o-c^aipa, a ball, and Sphcero- 

 ceras), his involute Nautilicone ; Serpenticone, Hyatt's Ammoniticone, 



