568 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



difficult to distinguish even ventral valves from the genera Reticularia and 

 Martinia. It is only by a careful system of comparison of the poor with 

 more perfect specimens that the proper affinities can be ascertained. 



The greatest difficulty is experienced in the case of Seminula humilis, 

 especially when the material is scanty and poorly preserved. 



Unfortunately, this is the usual condition. The characteristic surface 

 ornamentation renders the genus Cliothyris peculiarly liable to exfoliation. 

 Usually the spines have been broken away, their base giving the lamellae a 

 scalloped appearance. When the shell is entirely gone, casts of the interior 

 show only a few heavy growth-lines. When exfoliation has obliterated sur- 

 face characters, reliance must be placed upon other peculiarities in distin- 

 guishing CI. crassicardinalis from Seminula humilis. The shell of the former 

 is less convex and the beaks are more attenuate. These characters do not 

 always afford satisfactory results; yet I am unable to designate others. 

 The two species occur together at several localities, notably from the upper 

 part of bed 27, Crowfoot Ridge, Gallatin Range, but it so happens that 

 they are there especially well preserved and their distinctive characters 

 shown in the clearest manner. 



Where the area is concealed, exfoliated specimens might well be taken 

 for the young of Martinia rostrata or the reverse, but the latter will be seen 

 to have a more prominent beak, and to be provided on the ventral valve 

 with a shallow but perceptible sinus reaching to the extremity of the beak. 

 Reticularia cooperensis is more transverse and the ventral valve more 

 elevated. 



Formation and locality: Madison limestone, limestone bluff north of 

 Little Sunlight Creek, Absaroka Range, 600 feet above the stream; Arnold 

 Hague. Upper Gallatin Valley; divide between Gallatin River and Panther 

 Creek, Gallatin Range; east face of Antler Peak, Gallatin Range; W H. 

 Weed. Crowfoot Ridge, Gallatin Range, top of bed 25; J. P. Iddings and 

 G. M. Wright. Same, upper part of bed 27, bed 28, bed 29, bed 31; J. P. 

 Iddings and W. H. Weed. Summit of peak west of Antler Peak, Gallatin 

 Range ; south slope of same ; summit of Three River Peak, Gallatin Range ; 

 south base of Quadrant Mountain, Gallatin range; cherty belt, Bighorn 

 Pass, Gallatin Range; J. P. Iddings. Crowfoot Ridge, Gallatin Range, bed 

 24; A.C.Gill. North of Owl Creek, northeast slope of Teton Range; W. H. 

 Weed. Kinderhook age, Burlington, Iowa. 



