CTRTOCEEATID^. 
265 
which lead by gradation from arcuate, C 5 Ttoceran forms to close 
coiled nautilian shells, the whole showing a connected series of 
transitions in the form and outline of section, sutures, structure and 
position of siphon, and shell ornaments and apertures. In some 
cases these graded series are in accord with the chronological record, 
the straight appearing first, the arcuate either in company with 
them or later in time, and the gyroceran and nautilian latest .... 
The generic terms Cyrtoceras, Gyroceras, and Nautilus are really 
only descriptive terms for the different stages in the development of 
an individual, and also the different stages in the development or 
evolution of the series of adult forms in time. In other words, each 
of these genera as now used, include representatives of all the 
different genetic series of Tetrahranchs which are either young shells 
in the corresponding stage of growth, or adult shells in the corre- 
sponding stage of evolution.” 
With reference to the retention of the words Cyrtoceras, Gyroceras, 
and Nautilus in a generic sense, I can see no reason why they 
should not continue to he so used, if the genera they represent be 
properly restricted. Prof. Hyatt employs the name Nautilus in a 
restricted sense, why therefore should we drop the other two out of 
our nomenclature ? 
The following diagnosis of the typical Cyrtoceras is here drawn up 
in order that the species belonging to it may he more readily dis- 
tinguished from those of the two sections I have adopted as sub- 
genera, viz. Meloceras and Ooceras. 
Genus Cyrtoceras (restricted) : — Includes rapidly expanding, usually 
large shells, which are of a depressed-elliptical form in cross section. 
The siphuncle is external, marginal or suhmarginal, often with radi- 
ating deposits. The septa are slightly concave, and the sutures 
point towards the aperture on the outer or peripheral side of the 
shell. Type, Cyrtoceras dejyressum, Goldfuss h 
Cyrtoceras depressum, Goldfuss ^ 
1832. Cyrtocera depressa, Goldfuss, in von Dechen’’s Handb. der 
Geognosie von H. T. De la Beche, p. 536. 
^ The following species, in addition to the type, may be enumerated as 
examples of this genus ; — Cyrtoceras Llandoveri, Blake, C. Turnus, Barrande, 
C. hospitale, Barr., C. nigrum, Barr., Silurian ; C. multiseptatum, Boemer, 
Devonian. 
^ The real author of this species is Broun, for he was the first to describe 
and figure it ; but as all subsequent writers agree in assigning it to Goldfuss, a 
change in its authorship at this late date would be attended with considerable 
inconvenience. 
