lvi 
SYNOPSIS OF GENERA. 
Amnigenia, Hall. (Pal. N. Y., vol. v, pt. 1. Plates and Explanations: 
PI. xl, tigs. 1-4; pi. lxxx, fig. 12). 
[Type, Amnigenia Catskillensis, Vanuxem, sp.] 
Shell equivalve, very inequilateral, elongate, sub-elliptical or narrowly sub- 
obovate. Anterior end very short, the margin rounded. Posterior end pro¬ 
longed and wider than the anterior, the extremity rounded. Beaks sub- 
anterior, low and appressed. Cardinal line gently arcuate. Umbonal slope 
convex, not defined. 
Surface marked with concentric striae of growth, which become lamellose on 
the posterior portion of the shell. An obscure, broad depression extends from 
anterior to the beaks, obliquely to the ventral margin. 
llinge-line long, marked by one or two lateral folds parallel to the dorsal 
margin. Other characters of the hinge unknown. Ligament strong, external, 
extending more than half the distance from the beak to the posterior end of 
the shell. Anterior muscular impression large and prominent, situated just 
anterior to the beaks. Posterior muscular impression large, shallow. Pallial 
line simple. 
The specimens of the type species vary very much in form and proportions 
from the conditions under which they have been imbedded in the matrix. The 
specimens illustrated on plate xl, figs. 2, 3 and 4, and plate lxxx, fig. 12, 
although laterally compressed, appear to retain nearly their natural form and 
proportions. The specimen fig. 1, of plate xl, represents a very common con¬ 
dition of the fossil when the shell has been vertically compressed and the propor¬ 
tions are distorted. 
This genus does not possess characters which properly ally it with any of the 
palaeozoic marine families. From its occurrence with the remains of land plants, 
and the absence of brachiopoda, and other marine life, in the rocks where it is 
found, it would seem that the single species yet observed had its habitat in 
fresh or brackish water, and is a primitive representative of the comparatively 
recent genus Margaron. 
Type and example: Amnigenia Catskillensis , pi. xl, figs. 1-4 ; pi. lxxx, fig. 12. 
