306 
PALAEONTOLOGY OF NEW YORK. 
account of this stability in its features it is difficult to establish any satisfactory 
subdivision of its members, especially since the genus has been left more com¬ 
pact by the recent elimination of some of its aberrant forms De Koninck 
proposed* a classification of the species into five sections, based upon the 
nature of the external ornamentation, as follows: 
I. Concentrica, those with concentric folds or undulations, like C. concentrica. 
II. Comata , those with more than one hundred smooth radiating striae. 
III. Striata, those in which the striae are less than one hundred and more 
than thirty. 
IY. Plicosa, those with less than thirty striae. 
Y. Rugosa, those with rugose radiating plications. 
An additional group was proposed by Mr. Davidson, viz., Laves, to include 
smooth shells, like C. polita, McCoy, C. glabra, Geinitz, etc.; and Waagen has 
more recently added another, Grandicostata, for species with very strong and 
high radiating ribs. Such an arrangement as this is of course quite conven¬ 
tional, and can not meet the requirements of an exact classification, though it 
may still serve a useful purpose in the absence of a better one. Of the first of 
de Koninck’ s sections, Concentrica, we have no representation in American faunas. 
The second and third were properly united by Waagen, and will include the 
great majority of all known species; the Plicosa may embrace such forms as 
C. mucronata and C. lepida, Hall; of the Rugosa and Grandicostata, we have no 
representatives. The Laves are a group characterizing the Carboniferous and 
Permian, of which we have the species C. glabra, Geinitz,f while C. polita, 
McCoy, occurs in the Carboniferous throughout Great Britain, and Waagen 
has described^ five additional species of this type from the Productus-limestone 
of India. 
The genus Chonetes presents many points of structure in common with 
Plectambonites. This fact is best seen in the usual size and general contour 
* Monographic des genres Productus et Chonetes. 1S47. 
t The species C. glabm, Geinitz, and C. Iwvis, Keyes, are synonymous; the former having precedence 
in time must stand, since the C. glabra, Hall, has been shown to be identical with C. lineata, Vanuxem ; Pal. 
N. Y„ vol. iv, p. 121. 
| Salt-Range Fossils, vol. i, pi. iv, pp. 616, et seq. 1884. 
