296 
PALAEONTOLOGY OF NEW YORK. 
slight divergence, thus forming the inher 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^ena 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. Plectambonites planissima. After Pander. 
founders of these groups. The first two of Dalman’s four species of Lept^na 
are synonymous with 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 Leptaena. 
The genus Plectambonites was thus defined by its author: 
“ Die Riickenflache ist wieder dreieckig, und wiirde man nach ihrer Gestalt 
allein sich richten, so miissten die hielier gehorigen Formen unmittelbar auf 
die Orthamboniten folgen, allein wir mlissen zu gleicher Zeit die ganze Gestalt 
der Muscheln betrachten, und so sehen wir, dass diese, hauptsachlich durch 
die starke Concavitiit der Unterschale, riaher an Productus kommen. Die 
Riickenflache obgleich sie im Allgemeinen dreieckig, und anfangs ziemlich 
