280 PALEONTOLOGY OF NEW YORK. 



TjBNA. In 1844, McCov, having proposed to apply the term Lept^na to species 

 of Prodcctus or Chonetes or both, suggested a new term, Leptago.ma, for sliells 

 like Producta analoga, Phillips -= Leptczna rhomboidalis* 



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

 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 whether 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 internal 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 LEPTiENA to this peculiar group of forms, 

 it will l)ecome 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 is no name among all those which have been suggested or superceded which is 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. 



* Db Vbrnbdil, in 1845, proposed a classification of the species then I'eferred to this genus, on the basis 

 of their superficial ornamentation. His conception of the g-eniis is expres-^ed in his own words : " Nous 

 proposons de rendre au genre Lkpt.sna, sa valeur primitive, en y reunissant toiites les coquilles plus ou 

 moins analogues a celles que Dalman avail ain.si nommt'es " (Gi^ologie de la Russie, etc., p. 21.i). We Ihore- 

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

 some othei-s. 



t See General Summary, p. 370. 



