503 
Thus, although both Barrande and Hall have courteously ac- 
knowledged each other, and have mutually joined names as au- 
thority for this genus calling it Trochoceras, Barrande and Hall, 
the Trochoceras of the former is not the same as that of the latter, 
and the name of one or the other or both must be dropped. 
I have therefore retained Trochoceras Barrande, and propose for 
Hall’s remarkable forms, Zvochoceras Gebhardi and turbinatum, 
the new name of Mitroceras* with Mitroceras (Zvoch.) Gebhardt, 
sp. Hail, Pl. Ixxvii and Ixxviia as the type. 
It must not be supposed that all forms of Nautiloids having the 
turbinate spiral are devoid of impressed zones. There are some 
species that do have this characteristic, but it is invariably slight, 
and occupies necessarily a position on the sides rather than on the 
dorsum of the whorls. 
Lituitide. 
Recent investigations have shown that this group, instead of in- 
cluding about all of the unrolled, shell-covered Cephalopoda of the 
Paleozoic, must be limited to certain well-defined homogeneous 
series with peculiar characteristics. 
My observations lead me to think that Lituites is a degenerate 
form of Cyclolituites, a view similar to that of Holm and Schréder, 
who regard this genus as the radical of the Lituitide corresponding 
to the younger stages of true Lituites. 
The genera included in this family form a degenerating series 
which may have evolved from Cyclolituites, or some form that this 
most closely represents, becoming specialized by reduction of the 
spiral and simplification or loss of correlative characters during 
growth of the whorl, lessening curvature of the annuli and lines of 
growth and in the outlines of the apertures, until finally, in the ex- 
treme forms of Rhynchorthoceras, the whole shell is straight or 
orthoceran, except during the earliest stage, the nepionic, and in 
that it is not a perfect coil. 
This process takes place through the disappearance in the earlier 
' stages of the progressive characters of Cyclolituites and the gradual 
replacement .of these by characteristics that first appear in the 
paragerontic stages of such species as Ang. precurrens. ‘That is to 
say, Rhynchorthoceras has from a comparatively early stage the ven- 
*From Vf (Tpa, agirdle, but also used for ‘‘ turban,” in which sense it is here quoted. 
