MOLLUSCA—PELECYPODA. 435 



Differs from Mytilarca in its less oblique form and in the absence 

 of teeth. Ordovicic. 



195. C. lamellosa Hall. (Fig. 569, a.) Ordovicic. 

 Anterior margin almost straight and sloping sharply back- 

 ward. Beaks attenuate. Concentric growth lines lamellose. 



Stones River Group : Illinois, Wisconsin ; also Minnesota, Okla- 

 homa. 



196. C. undata Emmons. (Fig. 569, b.) Ordovicic. 

 Form subquadrate. Surface crossed by broad undulations. 

 Trenton : New York, Middle Galena of Minnesota. 



LXI. Aviculopinna Meek. 

 This differs from Pinna in that its beaks are farther removed 

 from the extreme point of the shell though still nearly terminal ; 

 this leaves a small anterior ear, 

 best seen in internal molds. A 

 linear thickening is present along 

 the cardinal border. Surface 

 marked by strong, regular, equally 



FlG. 570. Aviculopinna americana. 



-distant, concentric lamellae. Car- (Kansas Pal VI.) 



bonic and Permic. 



197. A. americana Meek. (Fig. 570.) Carbonic. 

 Beaks nearly obsolete, very oblique, and but slightly behind the 



obtusely pointed anterior extremity. Surface marked with two or 

 three broad, faint, radiating ridges on the posterior dorsal region. 

 Coal Measures : Ohio, Iowa, Missouri, Nebraska, Kansas. 



198. A.? peracuta Shumard. (Fig. 571.) Carbonic-Permic. 

 Shell almost cylindrical. Cardinal edges of valves suddenly 



erected so as to give the hinged margin a keeled appearance. 

 Surface covered with very obscure growth lines. Differs from the 

 typical Aviculopinnas in its non-lamellose surface. 



Throughout the Carbonic from Pennsylvania to Colorado and 

 Arizona. Also in the Permic of Kansas. 



LXII. Pinna Linnaeus. 

 Shell equivalve, thin, with a long hinge line. Beaks very 

 nearly or quite terminal. Valves keeled, triangular, wholly open 

 and truncate behind, without teeth. Muscle scars two. Shell 



