306 PALEONTOLOGY 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. StriatcE, those in which the striae are less than one hundred and more 



than thirty. 



IV. Plicoste, those with less than thirty striae. 



V. 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 Rugosce 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 fi'om 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 



* Monog^-aphie des genres ProducUis et Vhoiietes. 1847. 



t The species C pte&ca, Geinitz, and C. Jwvis, Keyes, are synonymous ; the former having- pi-ecedence 

 in time must stanj, since the C. glabra. Hall, haa been shown to be identical with C. liniala, Vanuxem ; Pal. 

 N. Y., vol. iv, p. 121. 



t Sttlt-Rang-e Fossils, vol. i, pi. iv, pp. 61(5, et seq. 1884. 



