428 NORTH AMERICAN INDEX FOSSILS. 



and basal margins regularly rounded. Surface crossed by con- 

 centric striae and usually by radii. Teeth absent. Pallial line 

 simple. Siluric and Devonic. 



177. L. curtum Hall. (Fig. 556, d, e.) Devonic. 

 Length usually less than height. Surface marked by 25-30 



regular plications with wider interspaces and by fine lamellose 

 concentric striae. 



Marcellus and Hamilton : New York. 



178. L. ornatum Hall. Devonic. 

 Differs from L. curtum in its proportionately shorter and 



straighter anterior margin and the more numerous plications 

 (45-60). 



Hamilton-Chemung : New York. 



179. L. acutirostrum Hall. (Fig. 556,7'.) Devonic. 

 Acuminate ; lateral margins long. Surface marked with simple 



radiating plications crossed by coarse concentric growth lines. 

 Genesee : New York. 



LII. Pteroch.enia Clarke. 

 Differs from Lunulicardium in its thin valves, surface marked 

 only with fine concentric growth lines ; all radiating lines lie on 



the inner surface of the valves and 

 may be outwardly visible by trans- 

 lucence. Beaks turned forward. An- 

 terior to the beaks and bounding the 

 conspicuous byssal opening is a wing- 

 like flange which is widest at the 

 beak becoming rapidly narrower to- 

 wards the base of the shell. Devonic. 



180. P. fragilis (Hall). (Lunulicar- 

 dium fragile^) (Fig. 556,/-/.) 



Devonic. 



Fig. SZq. Pterochcznia sinuosa, c , ,. ,. -, • u • ,. 



. * , *' , Ar n , , Small, not exceeding J- inch in di- 



nght valve, X 3- (After Clarke, ' to z 



N. Y. Surv. Mem., 6.) ameter, elongate-oval, fragile. Sur- 



face marked with concentric striae. 

 Usually the flat expansion bordering the posterior edge is broken 

 off. 



Marcellus-Portage : New York-Indiana. 



