r.hullUM.) 



right \.iht- in which a rm\ ..(' minute punct.i- i- gnu-rail y distinguishable; overlap 



'I around (In- ' ..-iitr.ill;. ' in ran 



neither valve ha- a flange or flattened l.order. and when pre-ent it i- in all ca-e- 

 narrow and undefined; ilo: " -omewhat t hickened. e-p.-cially ni"" 1 the left 



-i.l.-. Surface of valve- smooth or .lintly pitted, rather evenly rouvex 



witli th. -t thickne -mien hat beneath the center, a low ridge-like tlucken- 



in- along the po-terinr half <>f tin- d'.r-al margin of the left \.ilve i- to lie noticed. 

 6 ju-t di-tiiigai-hal>le in m< rely *o di-tinct a- in (lie specimen 



tijjun'.l, ofton not to be detected. <>n the inner Airfare however it is always marked 

 distim-t pit. Mu-i-le -pot not di.-t in^ni-halile externally except when the 

 specimens are \v I. Imt on the inner -ide it i- often well marked and -urrounded 



l.y line reticulating radial line-, short dorsally. longest p Tally. On the 



inner side of theventr oftheri^ht valve there are two row- of small papilla'. 



three to five in each, the numler seeming to increase with atfe. The purpose of 

 these papilla 1 , one series of which occurs in the anterior third, the other in the pos- 

 terior, evident 1. . preuMit. undue overlapping of the valves l>y presenting an 

 ol-tacle to the entering vend of the left valve. 



of tlii- I have l.efort- me i hundred -pecimens, represent- 



ing twelve localities in the states of Minnesota. \Vi-con-in. Illinois. Kentucky and 

 Tennessee. Considering its wide geographical range and abundance, it is reraarka- 

 l.ly con-taut in all it- character-. T'n.n i( :- so in its outer form is clearly enough 

 >hown liy theali.ixe moasurement>i. taken from representative examples. They show 

 further that the northwestern specimens are on an average about one-third larger 

 than those from Tennessee. In all other respects however they are all practically 

 identical. 



In 1- (//.) I Relieved it prol.al.le that /. -./, hi. m<i -Jones, would prove 



distinct from I.. t.il.uHt, s Kut it i- now .jiiite evident to me, as it also has become to 

 1'rof. -lones. that there is no ground whatever for any distinction between them. 

 I'ompared with other the I'pper Silurian L. h Schmidt, is not far 



removed, and t : ilmlinu from l.-ike Winnip. . -imilar indeed. Still 



as pointed out l>y Prof, .lone- t hei '>f minor ilitl.-rence- lietween them, 



and these will no douht he ad'led to when the later form i- fully known. An. 

 closely related form is the /,. uilm imiidt. from the I'pper Silurian of Hi; 



It- hinge line is shorter and the antero ventral curve somewhat fuller, hut in other 

 respects, even to the rows of punctae along the ventral margin, the tuo specie- 

 much alike. /.. //mi,/// I' 1 rich from the I'ppcr Trent.m of Kentucky i- more ol.li.|iiely 

 produced posteriorly and ha> a -horter hinge line, a tl.r :itral edge, di-tinct 



tlanges. and Letter developed tul- 



