188 CllETACEODS TEKIOD. 



Family C0EBULIDJ5. 



Genus GOEBULA Bruguifere, 1792. 



Corbula nematophora Meek. 



Plato XVII, fig. 7 a, h, ami c. 



Corhila nemato;phora Meek, 1872, Geol. Surv. Montana, Iclabo, Wj'oming, and Utah, 49G. 



Shell small, subovate or subtetrahedi-al iu marginal outline, nearly 

 equivalve, moderately gibbons, height in relation to the length about as 

 seven to ten, posterior extremity somewhat produced; basal margin broadly 

 rounded, abruptly rounded up at each end to meet the anterior and pos- 

 terior slopes respectively; anterior slope more abrupt than the posterior, 

 almost straight or slightly concave; posterior slope longer and more oblique 

 than the anterior, nearly straight; umbones rather prominent; beaks small, 

 slightly incurved; umbonal ridge somewhat distinct; postero-dorsal region 

 flattened and sometimes marked by an inconspicuous radiating raised line. 



Surface marked by fine, raised, concentiic lines of growth, and also 

 by strong, concentric folds placed at irregular intervals, having apparently 

 been produced by the temporarily an-ested marginal growth of the shell at 

 irregular intervals of time. Sometimes the surface, especially that of the 

 left valve, is nearly free from these folds, but in other cases they are so dis- 

 tinct as to produce considerable distortion of the valve. 



Length, six millimeters; height, foui* millimeters. 



The average size of the specimens contained in the collections is con- 

 siderably less than that given by Mr. Meek (Joe. cit.) for his largest typical 

 example; but there seems to be very little reason to doubt that the shells 

 under examination are specifically identical with G. nematopliora. 



The average size of the specimens obtained at the second locality 

 named below is greater than that of those obtained from the first-named 

 locality. This is probably due to the difference in the conditions of the 

 habitat of the mollusks at the two locahties respectively, while they lived, 

 as indicated by the composition of the rock in which they are now embed- 

 ded; that of the first-named locality being comjjaratively pui'e limestone, 



