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to reach the extreme point of degeneracy, so as to allow the aper- 

 ture to become again free and complete on all sides. 



This is, of course, negative evidence and it may be, as in Ammon- 

 oids, that the dorsal edge of the mantle never loses in any series 

 when restored to freedom the power to resume the shell-secreting 

 structures and function on the dorsal side. It can be readily seen 

 that as the whorl became gradually loosened from the inner whorl 

 the mantle border would extend the secreting furrows inwards from 

 both sides, or, more correctly speaking, perhaps, the non-secreting 

 edge of the dorsal border would be contracted and finally disappear. 

 There is no antecedent improbability that this might not take place 

 in any involute nautilian shell at a sufficiently degenerative substage 

 of its ontogeny. The remarkable fact, however, remains that it does 

 not take place so far as I know, although I have constantly been on 

 the watch for some such examples. 



Ammonoidea. 



It is not necessary to give any extended notices of observations 

 on special groups in this order. I have already described the 

 absence of the impressed zone in the ordinal radicals Bactrites and 

 in most of the Nautilinidse on pp. 361, 362, 411, 413 and the 

 figures and explanations of PI. ii, and Figs. 40-42, PI. viii, of 

 Mimoceras lituitm. The more specialized genera of the Goniati- 

 tinae have the impressed zone, but it is strictly a contact furrow and 

 appears as shown in figures of Agoniatites fecundits, one of the Nau- 

 tilinidae, sometimes very late in the ontogeny. In other still more 

 highly specialized species the loose coiling of the young, figured by 

 Sandberger in several species of Gephuroceras, Manticoceras lati- 

 dorsale of the Devonian and by the author in Glyphioceras crenis- 

 tria and atratus of the Carboniferous* indicates, that this zone is 

 either absent on the ananepionic dorsum, or, if present, must occur 

 as a slight dorsal furrow due to tachygenesis. The larger number 

 of the Goniatitinas, as shown by Branco and the author, have, how- 

 ever, such closely coiled nepionic stages that, as in all Ceratitinae, 

 Lytoceratinae and Ammonitinae, so far as known, the umbilical per- 

 foration is closed along the mesal line as shown in Fig. 3, PI. iii, 

 and is represented only by funnel-like lateral prolongations, which 

 do not appear to have an open connection with each other. 



* Embryology of fossil Cephalopoda, PI. iii. 



