531 
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 Cyréo- 
ceras liratum. Wall 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 Cyrfo- 
ceras liratum. 
RHADINOCERAS HYATTI. 
MADTILUS HYATE, Elall (Ped. Vv. Y.,-v, Pt. i, Pl. exxvi). 
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 cornudum, but it is a depressed ellipse in 
the nepionic stage, increasing more rapidly by growth in its trans- 
verse diameters than in cornudum. ‘he 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 Cosstl 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. 
