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eralized Goniatitinae.* The higher Goniatitinae and almost all 

 shells of the remaining suborders of this order have a rostrum on the 

 venter. Shells having this peculiar structure, due to the absence of a 

 hyponome, continue to increase or broaden out the bands of growth 

 after the rostrum is introduced into the ontogeny, producing often 

 long-pointed or palmate growths. This is certainly independent of 

 the spiral mode of growth and has no effect upon it, since the ros- 

 trum is very well developed in forms like Baculites, having phylo- 

 gerontic straight whorl, and it may be entirely absent in the geron- 

 tic stage of Ptychoceras and in forms with lateral lappets to the 

 apertures as in some Scaphites and other genera which are more 

 closely coiled. 



Taking into consideration all of such facts there still remains a 

 certain obvious and necessary relation between the ratios of growth 

 of the bands on the outer and inner sides of a coiled shell which 

 has been described above, and which is a mechanical necessity of 

 growth in a spiral. 



It is also true, as a rule, that the lateral diameters increase faster 

 in shells with small umbilical perforations than in those with large 

 open centres. But this seems to be merely a function of the quicker 

 growth and general accompaniment of the early age of such types and 

 to have direct exceptions that do not enable us to bring it under any 

 uniform law. Thus Estonioceras is a type with large umbilical per- 

 forations and slow-growing ventral bands of deposition, but the 

 lateral diameters increase fast as in the young of some forms like 

 Estonioceras imperfectum, Figs. 20 and 21, PL vii. 



In nearly all shells there is a noticeable tendency to decrease the 

 lateral diameters in the later nepionic and neanic stages, and is 

 obviously due to Minot's law of growth, which is noticed in the In- 

 troduction, p. 381. 



Among Nautiloids it is observed in Trocholites as a generic 

 character occurring in the neanic stage and is in these species and 

 in the nepionic stage of Ammonoidea an absolute decrease so well 

 marked that in the former the apex and in the latter the protoconch 

 are not covered and can be seen beyond the outer volution, this 

 being the usual aspect in a ventral view of a Goniatite or the young 

 of Ammonitinae. 



A similar decrease occurs in other forms of Nautiloids than 



* See Introduction, p. 355. 



