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which may be a survivor of the ancestral genus of this group. I 

 allude to the peculiar arcuate species described by Hall from the 

 Goniatites limestone of Manlius, N. Y., under the name of Cyrto- 

 ceras liratum. Hall recognized the affinity of this shell, in the 

 ornamentation and form to species here described as included in 

 Rhadinoceras, and it can be easily observed that the young of 

 Rhadinoceras cornulum directly repeats the characters of his Cyrto- 

 ceras liratum. 



Rhadinoceras hyatti. 



Nautilus hyatti, Hall {Pal. N. Y., v, Pt. ii, PL cxxvi). 



This species, so far as figured by Hall and so far as known to me 

 from the observation of Prof. Hall's collection, is even less closely 

 coiled than Rhadinoceras cornulum. 



The early stages figured by Hall show no dorsal furrow and the 

 form is similar to that of cornulum, but it is a depressed ellipse in 

 the nepionic stage, increasing more rapidly by growth in its trans- 

 verse diameters than in cornulum. The affinity of this species with 

 the nepionic stage of Nephriticeras is indicated not only by the form 

 of the whorl, which is identical, but by the presence of coarser 

 longitudinal ridges, and by the sutures. 



Whether the whorls of this species were ever in close contact is 

 doubtful, on account of the absence of more complete specimens and 

 the want of a contact furrow on the fragments, so far as known to 

 me. 



But the single fragment figured by Hall, and examined by me, 

 was not old enough to settle this question, and I am inclined to the 

 opinion that it will be found to be a true nautilian shell. 



Nephriticeras. 



This genus, described by the author in Genera of Cos'sil Ceph- 

 alopods, p. 300, formerly included the transitional species separated 

 above under the name of Rhadinoceras. 



These shells are all unquestionably nautilian* 



The early part of the nepionic, probably metanepionic substage, is 

 similar in transverse section and ornaments to the full-grown shells 

 of Rhadinoceras, but the paranepionic volution becomes speedily 

 depressed and subtrigonal, the dorsum broad and much flattened, 

 the abdomen elevated and narrower than the dorsum. 



The siphuncle is dorsad of the centre. 



