584 
eralized Goniatitine.* The higher Goniatitinee 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 Jarge umbilical per- 
forations and slow-growing ventral bands of deposition, but the 
lateral diameters increase fast as in the young of some forms like 
LE stontoceras 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 Ammonitine. 
A similar decrease occurs in other forms of Nautiloids than 
* See Introduction, p. 355. 
