176 Natural History Bulletin. 



and numbers of them have been distributed to correspondents 

 so named; but the larger size, the differently shaped and dif- 

 ferently proportioned muscular impressions, the more nearly 

 parallel dental laminae and the finer punctation entitle this 

 form to rank as a distinct species. 



SCHIZODUS, King. 



SCHIZODUS SYMMETRICUS, N. S. 



PLATE II., FIGS. 3, a, b. 



Shell rather large for the genus, suborbicular in outHne, 

 length about one and one-eigth times the height; valves mod- 

 erately convex; beak somewhat prominent, incurved, situated 

 but a little forward of the middle of the sHghtly curved hinge 

 line; anterior margin regularly rounded and passing without 

 abrupt change of curvature into the rounded ventral margin; 

 posterior margin obliquely truncated and joining the ventral 

 margin in a short, abrupt curve; an obscure ridge extends 

 from the umbo downward and backward to the junction of 

 the posterior and ventral margins; shell somewhat concave 

 between umbonal ridge and posterior margin; in the cast a 

 distinct groove passes from the beak along the anterior bor- 

 der of the umbonal ridge, nearly half way to the basal margin. 



Height, a Httle more than one inch. Length, exceeding 

 one and one-eighth inches. Convexity about three-fourths of 

 an inch. 



This species resembles somewhat the form described as 

 Dolahra (?) alfina by Professor James Hall in Geological 

 Survey of loiva^ Vol. /., Part 2, page 716, Plate XXIX., Fig. 

 2, but the difference in the outline of the front margin and in 

 the width of the shell behind the beaks afford easily recog- 

 nized distinguishing characteristics. 



Found in the Upper Coal Measures at Braddyville, Iowa, 

 associated with Spirifera cajuerata, Sireptorhynchus robustus^ 

 Terebratula bovidens, Productus seniireiiculatus, P. symmetri- 

 cus, Chonetes gramilifera and other forms characteristic of 

 that horizon. 



