-mall. rounded. -ituated in tlio antero-dorsal angle. I 1 " and |..illi.il line 



undetermii: 



In ino-t -pecimen- there. -hree. thin. parallel and dl>lit|ue ridges on 



each side at the extremity of tin* hinge. The-e opposed sets of ridges are separated 

 by an interval in adult examples, but it i* sea: !>e qiie.-t ioned t liat in an earlier 



Stage in the development of th.- -hell they represented posterior lateral teeth similar 

 t< tho-e of < 'i/i't'iilnntn. Thi> fact mti-t have an important hearing upon the qiie 



nealogy, Imt. in the absence of any knowledge of similar types in earlier strata, 

 it i- not nou po-Mhle to .1; with anything like certainty of arriving at a true 



solution of the question. 



Tin- i- .1 well marked species and one that i- not likely to he confounded with 

 any of the associated shells. Its nearest congeners seem to be If. scofieldi and 1C. 

 uterlimjen.ti* M. and \\ . -p.. the latter particularly, but in both of these species the 

 umbonal ri.l^e is more <-on>pieuous and the outline different. The former again has 

 a longer escutcheon and hinge, much larger anterior end and more prominent 

 postero-cardinal angle, while in the latter the -hell i- more oblique, the posterior 

 angle narrower, the cardinal area murh wider and the Leak-- farther apart. 



Formation and locality. -L '<>n llrin>- \ isconsin and Minaoapollm Minn. 



\\nni .1.1 \ -ri.KiKi.iH I'lrirh. 



I-I.ATI 

 M'hitrlla fcofleldi UI.UH n. !" 1M| and 382. 



Shell of medium - y eonvex, moderately ol>lii|iie. -ul.t rape/oidal in 



outline, with the hinge line longer, -traighter and better defined than ii-ual. Ante- 

 rior end unusually long and wide, the outline gently rounded from tin- .-ular 



junction with the hinge line; basal margin slightly con \ ex. oblique, descending to the 

 .nded po-tero-ba-al angle; posterior mar-in .-ubtriincate. -lightly oblii,iit 

 and but litt' x in the upper half. I' inborn-- prominent, -ub.-arinate behind, with 



the beaks approximate, obliquely enrolled and -ituated a little more than one-third of 

 the length of the hinge line behind it- 1 ridge is a 



conspiciou- feature, although becoming obsolete before reaching the postero-baml 

 margin. Posterior to the ridge the -in ave; in front and 



ath it convex. Surface marked by rather <i .sth. 



of which the marginal ones may, in old e\ asMimc a -ub! . character. 



itcheon high Imt narrow in a dor-al view. 'riated longitudinally and 



extend ing anterior to the beaks. Internal liga >orts appearing as a double 



ridge in each valve beneath th : ior h.ilf of the e- erior liinge 



