PELECYPODA—PRIONODESMACEA. 477 



ties rounded. Beaks low and appressed. Hinge line long, gently 

 arcuate. Umbonal slope not defined. Surface marked by con- 

 centric striae which become lamellose on the posterior portion of 

 the shell. Teeth obscure. Hinge line marked by one or two 

 longitudinal lateral folds. Ligament external, extending more 

 than half way from the beak to the posterior end. Pallial line 

 simple. Lived in fresh water. Devonic. 

 303. A. (Archanodon) catskillensis (Vanuxem). (Fig. 637.) 



Devonic. 



Shell large, elongate-elliptical. 



Oneonta : New York. Catskill : Pennsylvania. 



Fig. 637. Archanodon catskillensis, X %> ( Pal - N - Y., V.) 



LXXXIII. Naiadites Dawson. {Anthracoptera, Salter.) 

 Inequivalve, obliquely triangular, very inequilateral. Anterior 

 end oblique, forming a small lobe anterior to the umbonal ridge. 

 Hinge line straight. Hinge plate striated, with an obscure car- 

 dinal tooth, that in the left valve being anterior, and that in the 

 right posterior. Umbos small, inconspicuous, almost terminal. 

 Byssal notch always present. Shell very strong anteriorly, fragile 

 posteriorly. Surface covered with flat concentric lamellae. Pallial 

 line entire, represented by a series of small pits. Brackish or 

 fresh water ; attached by byssus. Carbonic. 

 304. N. carbonarius Dawson. (Fig. 638, e.) Carbonic. 



Hinge line more than half the length of the shell. Beak in an- 

 terior fourth of hinge line. Anterior margin abruptly rounded ; 

 basal margin nearly straight, with a slight sinus ; posterior margin 

 broad and regularly rounded. Shell usually much distorted by 

 pressure. Length one inch or more. Type of genus. 



Exceedingly abundant in the Coal Measures of Nova Scotia and 

 Cape Breton. 



