﻿PI,AGIOLA 303 



Animal having the outer plls narrow in front and wid<' 

 behind ; inner gills wide in front, posteriorly free or united to 

 abdominal sac ; marsupium occupying the posterior part of the 

 outer gills, but not extending quite to the hinder end, consist- 

 ing of well-marked ovisacs, which are rounded below ; a dis- 

 tinct sulcus extends the whole length of the kidney-shaped 

 marsupium at the inside and outside at some distance above 

 its base ; mantle edge thickened and somewhat doubled, in some 

 cases toothed or fringed below. 



Type, Uiiio seciiris Lea. 



The genus Plagiola, as I have constituted it, contains a 

 small number of North American species having rather short, 

 triangular-ovate shells with high, but mostly compressed, beaks 

 and a strong, usually rather sharp, posterior ridge. In gen- 

 eral the female shell has a more or less distinctly developed 

 marsupial swelling. The marsupium is much like that of 

 Lampsilis and the higher organized Naiades consisting of dis- 

 tinctlv marked ovisacs in the hinder part of the outer gills. 



The genus is easily divisible into three well-marked sub- 

 genera, the typical Plagwla with a heavy shell, with highly 

 painted epidermis and wide hinge plate, the female shell 

 being smaller and more inflated than the male, and having a 

 well-developed marsupial swelling; Amygdalonaias with much 

 more delicate, highly pointed shells with a narrow hinge plate, 

 and Artonaias. a group of Central American forms, with 

 concentric sculpture and full colored, often wrinkled, epider- 

 mis. It may be remarked that occasional specimens in both the 

 first mentioned subgenera are dull colored, destitute of rays 

 and have slightly wrinkled epidermis. 



Kr;v TO sprctes oP Pi.aoiot.a. 



Shell painted with broken rays. 



Heavy, with wide hinge plate. P. sccuris. 

 Subsolid, with narrow hinge plate. 



Inflated, strongly truncated behind. P. elegans. 



Moderately inflated, rather solid. P. coniata. 



Scarcely inflated. P. donaciformis. 



Compressed. P- macrodon.. 



