280 PALAEONTOLOGY OF NEW YORK. 



T/ENA. In 1844, McCoy, having proposed to apply the term Lept^ena to species 

 of Productus or Chonetes or both, suggested a new term, Leptagonia, for shells 

 like Produda analoga, Phillips = Leptcena rhomboidalis* 



There are sufficient reasons for limiting the application of the term LEPTyKNA 

 to shells conforming precisely to the structure exhibited by L. rhomboidalis. 

 Dr. Davidson, in his later years, was evidently convinced of the validity of 

 this group.f Whether L. rhomboidalis shall be allowed to include all the forms 

 from Silurian to Carboniferous faunas which have the characters given in the 

 foregoing diagnosis, or whetlier the numerous specific and varietal names that 

 have been proposed shall be recognized in whole or part will depend upon one's 

 conception of specific values. At all events the type of intern.al structure 

 accompanied by the peculiar corrugated and geniculated exterior has proved a 

 very compact and resistant combination, a fact evinced by the mere possibility 

 of a question arising as to the specific identity of the various forms. It is 

 worthy of note that among the American representatives of this type of struc- 

 ture which have been studied, the extreme differentiation of the muscular area 

 as described is even more distinctly exhibited in the forms of the early Car- 

 boniferous than in those of the Silurian and Devonian. 



Believing that it will serve a good purpose in the taxonomy of these organ- 

 isms to thus restrict the generic term Lept^.na to this peculiar gi'oup of forms, 

 it will become necessary to arrange under another designation the much more 

 abundantly developed " Strophomenas " of the Silurian, exemplified hy Lep- 

 tana alternata of Conrad. These differ essentially from L. rhomboidalis, not in 

 their exterior features alone, but also in their interior characteristics ; and there 

 is no name among all those which have been suggested or superceded which is 

 applicable, or can be legitimately used, and therefore it becomes necessary to 

 propose a new generic term. 



* De Vernedil, in 1845, proposed a classification of the species then referred to this genus, on the basis 

 of their superficial tirnaminitation. His conception of tlie g-eniis is expre.s.sed in his own words : " Nous 

 proposona de rendro au g'eni-e Lkpt^na, sa valeur piimitive, en y reunissant toutes les coquilles jilus ou 

 moiii.«i analogues a celles (jue Dalman avait ainsi nommees" (Geologie de la Russie, etc., p. 215). We thei-e- 

 fore find in his list of twenty-thiee species not only the various types of .structure given by Dalman, but 

 some othei's. 



t See General Summary, p. 'Xi'^. 



