NOMENCLATURE OE STAGES OF GROWTH AND DECLINE. 17 



all other forms, especially in the devonian latisellate and triassic angustisellate 

 embryos, the tendency to become closely coiled, and to inherit the depressed 

 primary radical whorl of Anarcestes, produced the Goniatitinula, and affected 

 even the protoconch. The protoconch through heredity becomes depressed fusi- 

 form by lateral expansion in the Angustisellati, and the embryonic nautiloid 

 character of the first septum in the asellate forms and its tendency to form 

 a broad ventral saddle in the latisellate and a narrow ventral saddle in the 

 angustisellate embryo is correlative with this progression of form. 



The goniatitinula is a true larva, corresponding to adults within the order. 

 We use the term because it is the characteristic larval form of the Ammonoidea, 

 which was introduced at first among adult Goniatitinoe, and in the higher forms 

 of this group became, by acceleration, fused with the microsiphonula. 



The remarkable researches of Branco enable us to state that this progres- 

 sion in complication of the embryo in form and sutures has no counterpart 

 in the parallel series of any pre-existing series of adult shells, except among 

 Nautiloidea; consequently the angustisellate peculiarities of the ventral saddles 

 and deep lateral lobes characteristic of the latisellate and angustisellate em- 

 bryos of the Devonian and Trias were not due to inheritance from primitive 

 adult radicals, but were later modifications originating in the caecosiphonula 

 from close coiling. They were correlative with the earlier or accelerated 

 development of the depressed whorl, and the quicker growth in bulk of the 

 whorl. Similar tendencies have been observed repeatedly in different progres- 

 sive series of Nautiloidea. Thus, wherever we have been able to trace the series 

 of species from a straight, or loose-coiled, to a close-coiled nautilian form, this 

 as a rule has more complicated sutures. The universal result of such progres- 

 sive specialization among the adult forms of Nautiloids is closer coiling, due to 

 quicker growth in bulk of the whorl, and is accompanied also by the evolution 

 of a larger ventral saddle. It is not surprising that similar mechanical results 

 should follow in the septa of the embryos of Ammonoidea, when similar changes 

 in the mode of growth occurred through the accelerated inheritance of the 

 depressed anarcestian radical whorl, and closer coiling in the caecosiphonula. 



Branco has observed the shortening of the larval stages in the Latisellati 

 as compared with the Asellati, and the still greater acceleration of development 

 occurring in the Angustisellati, and the correlation of these with the general pro- 

 gress in complication of the sutures of the adults of the same divisions in time. 

 This confirms our previously published opinions of the relation of embryos 

 and adults, and also agrees with those here published regarding the inheritance 

 of the primary, radical, smooth form in the depressed embryos of Latisellati and 

 Angustisellati, and the correlative evolution of the sutures and coiling. 



The microsiphonula appeared in the Ammonoidea with the second septum, 

 in what is morphologically the second air-chamber when compared with Nauti- 

 lus, though actually the first existing in the apex of the true conch. This 

 microsiphonula is also an accelerated form, since the siphon becomes very rapidly 

 or even abruptly attenuated. The collar or distinctive organ of the siphon among 

 the normal Ammonoids was not formed until later, though the precise period 



3 



