296 PALEONTOLOGY OF NEW YORK. 



slight divergence, thus forming the inner boundaries for two elongate muscular 

 scars, which are less sharply defined on their outer margins. The muscular 

 area is rendered quadripartite by two short transverse or oblique posterior fur- 

 rows. Vascular impressions radial, sometimes digitate. Shell-substance 

 fibrous, sparsely punctate. 



Type, Pledambonites planissima, Pander. Lower Silurian of Russia. 



Observations. In applying the term Plectambonites to the fossils usually refer- 

 red to the genus Lept^na, as interpreted by Mr. Davidson in 1853, there is a 

 sufficient justification in the necessities of the case, and in the rights of pri- 

 ority. In this work the example of many of the earlier writers has been fol- 

 lowed, in applying the term Lept^na to the type of structure expressed in 

 Dalman's first and typical species, L. rhomboidalis. 



If the progress of knowledge requires a stricter interpretation of generic values 

 than was prevalent a generation ago, at least no injustice has been done to the 



Pigs. 23, 24, 25. Plectrimbonites planissima. After PANDEK. 



founders of these groups. The first two of Dalman's four species of Lept^na 

 are synonymous ^ith L. rhomboidalis; the third, L. euglypha, Hisinger, belongs 

 to the genus Strophonella, Hall ; the fourth is L. transversalis, Wahlenberg, a 

 well known and widely distributed representative of the genus Plectambonites, 

 Pander, but which, under the conception of the genus, as rendered current by 

 Mr. Davidson, has served as the type of LEPTiENA. 



The genus Plectambonites was thus defined by its author : 

 " Die Rtickenflache ist wieder dreieckig, und wiirde man nach ihrer Gestalt 

 allein sich richten, so mlissten die hielier gehorigen Formen unmittelbar auf 

 die Orthamboniten folgen, allein wir miissen zu gleicher Zeit die ganze Gestalt 

 der Muscheln betrachten, und so sehen wir, dass diese, hauptsachlich durch 

 die starke Concavitat der Unterschale, naher an Productus kommen. Die 

 Ri'ickenflache obgleich sie im Allgemeinen dreieckig, und anfangs ziemlich 



