CAEBONIFEKOUS PEEIOD. 153 



Genus BAKEVELLIA King, 1848. 



Bakevellia parva Meek and Hayden. 



Plate XI, fig. 7 a and h. 



Balcevellia parva Meek and Haydeii, 1858, Trans. Albany Institute, iv. 

 BalieveUia parva Meek and Hayden, 1864, Palseout. Upper Missouri, 57. 



Shell very small, obliquely subovate in outline ; valves gibbous, especi- 

 ally the anterior half of each ; cardinal margin straight, its length not quite, 

 equal to the full length of the shell, forming an angle with its axis of about 

 30° ; postero-dorsal region compressed, subalate ; antero-dorsal region bluntly 

 prominent ; ventral margin broadly rounded downward and backward ; 

 posterior margin abruptly rounded below, and straight or slightly concave 

 above, inclining a little forward, and forming an obtuse angle with the 

 cardinal margin. 



Surface marked by fine concentric striae. 



Length of the largest specimen, six millimeters ; height, about four 

 millimeters. 



The collections contain examples from two widely-separated localities. 

 Those from New Mexico are from true Carboniferous strata, and differ 

 slightly from the types and description of Meek and Hayden. Those fi-om 

 Arizona are from strata probably of the Permian period. A figure of each 

 is given on Plate XI, which exhibit considerable difference in outline. This 

 may perhaps be due to specific difference ; but it ' is thought it is not 

 necessarily so, as other examples seem to show intermediate forms. The 

 type-specimens of Meek and Hayden were obtained from strata, regarded 

 by them as Permian, near the mouth of the Smoky Hill Fork of Kansas 

 River, Kansas. 



Position and locality. — Strata at the summit of the Carboniferous series ; 

 Bear Spring, Camp Wingate, New Mexico; and also near Jacob's Pool, 

 Arizona. 



