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lines of growth and the dorsal sutures are not correctly given. The 

 dorsal lobes exactly coincide with the impressed zone in Fig. 6 of 

 his plate. This figure shows the last part of the paranepionic volu- 

 tion in section below and the ananeanic with the impressed zone 

 above this. The smoothness of the impressed zone in Hall's Fig. 6 

 of this species shows that the longitudinal ridges Were obliterated 

 as they are in other forms by the pressure of the growing whorl, 

 and that this zone is probably due to contact and did not occur on 

 the free side of the volution in the umbilical perforation. I use 

 the general term "impressed zone," because, although my notes 

 and Hall's observations and the figures all seem to warrant the 

 statement that this zone in this species is a contact furrow, I have 

 not been able to revise and confirm these observations. 



Nephriticeras bucinum. 



Nautilus bucinus, Hall {Pal. N Y., v, Pt. ii, PL lx). 



The paranepionic volution is shown in Hall's Fig. i, PL lx, with 

 a convex dorsum, and in PL cvii, Figs. 2 and 3, it is again shown 

 with the siphuncle dorsad of the centre and the outline distinctly 

 subtrigonal. These figures indicate great variability in the time 

 at which the impressed zone appears, since the section in PL cvii is 

 very much larger than that of about the same age of Fig. 2, PL lx. 

 One is disposed to think that these are perhaps different species. 

 Fig. 2 of PL lx gives in front view a section of the paranepionic 

 volution with a distinct but narrow impressed zone marked on the 

 dorsum. This whorl has a nephritic outline and is very different 

 from the subtrigonal outline of a whorl with convex dorsum referred 

 to above, which belongs to an obviously later stage of growth in a 

 larger species. 



Having examined these specimens in Prof. Hall's collection some 

 years since, I find in my notes the statement that " no depression 

 (meaning the dorsal furrow) occurs in the centre of any of these 

 shells until the whorls touch, which they do at a late stage of 

 growth." The form changes from a depressed oval in the metane- 

 pionic to nephritic more rapidly than in Nephriticeras sublirdtum 

 and the transverse diameters increase faster. The longitudinal 

 ridges are smaller and less prominent than in Nephriticeras liratum. 



The sutures in the young have ventral and dorsal saddles and 

 only in later stages these are replaced on the dorsum and venter by 



