Ivi SYNOPSIS OF GENERA. 



Amnigenia, lliill. (Pal. N. Y., vol. v, pt. 1. Plates and Explanations: 



PI. xl, ligs. 1-4 ; pi. Ixxx, 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. 



Hinge-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 Ixxx, 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 

 palajozoic 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. Ixxx, fig. 12. 



