HI ATA. 



WbllrlU ..-ntrl.-.*. 1 



angular or *harply rounded and distinct .|iiite to the po-tcio-liasa! margin. An 

 obscure furrow in the middle of the ll.it cardinal slope. From II il.ill. -p. 



which -coins to be its nearest convener. it differs principally m the greater *harpnett 

 ami prominence of tlic ninl.onal rid^e. The anterior end i* much too small and 

 short for II'. rn<j<i(in<i. I! ./iY/</i. In each case other differences 



mi^ht In- mention--,!, hut those selected will, it is believed, sutlice. 



atest length. -''< nun.; di-tance from beaks to posterior extremity. _'_'..') mm.: 

 rior hitfht. l'.."i mm., thickne . \'.\ mm. Antero-ventral postero-cardinal diam- 

 eter, 17 mm. 



- taken from a doubtful left \alve. obtained from the lower Trenton 

 in.lola >unty, Illinois. ow.th; MMI rcpr'>fiit- an ancestral 



form or variety of the present - ''rom which al>o II'. >;-ntrirus(i, II'. tnttiniln and 



perhap- nthei >peciesas well have been evolved. More and better material, however, 

 i- n'ce*-ary In-fore -\\c\\ a \ iew of it- relations can be consdered either as proved or 

 ili>pro\ 



mitinii unit ln,-,iiiti/. '1 . tin- nil '. Minnesota. 



\Viim.u. \ .-,I>A 



IK XI. I. i 



nilia rrntrieo*! HALU 1847. P:t !. p. 16&. 



l:il-rarra tWK/HcoM, HALL, \K> III, p. 271, and TwHfth U<-p SUtc Cab., 



Ii|>. 10, 68 ai -<^rtod' - ni> Itn : 



riruttu HAI.L, !-' . - n..r WIIITMKLD, 188*. 



3W. (-' 



Helievin^ that this species i> repre-ente.l amon^ the undetermined fragmeir 



l- from Minnesota, I thought it well to give illustrations of authentic specimens 

 from the Trenton of New York. These were received in an exchange some time 

 ago. l^uite recently I sent two of them to I'rof. K. I'. \Vhitlield. of the American 

 MuMMim of Natural II A ho com | tared them with the original types of the 



species and verilied the identification. 



An examination of the New York examples established what I had already 

 <u.-pected from the original ti^ure>. namely, that the species is a true Whilelln and 



inmonly ltelie\ ' 'i/H<Ii,n/,i. Its place in the ft 



will' be seen at on compared with other species of the genus figured on 



plate* xi. and u.i. The shell was thin, the beak- \\ere full and prominent 

 iimlional riil^e. though not as -harply pecies of the gonu>. 



,11 a more conspicuous fi-.itun- than in an , - of ('i/i-t'l'>ntn. the hinge baa a 



narrow external lijMinental area or e-cutcheon, and rid^e-like supports fora posterior 

 internal ligament. Imt no posterior lateral teeth. In short, the species presents 



