08TRAOODA 



T-iruilrlla .,u.|rlllt.. 



includes /.'. tni'inloi'1,,1. /.'./ ;Wr.;/,/. /.'. /v ; /i/,/r/\ etc. i denion-t rat inn of t lii- 



line of development may he established ;iln'aly from known species. Compare. for 

 instance. /'. r,i/i<!'t\ Kmiiion- -p.. LoWtt Silurian, ami l'-i/ri<-lti,i ,-/,,,-l.i .lone-. /' 

 li>illi. .lour-. /,'. li'isnyltipln,-'! Krau-e. /.'. >> Hall, and l\f.,-l,ni-i fcftMMM Kr.m-c. 



I'pp.T Silurian, ami it i- clear that tin- change from the lir-t in tin- la-t \\.t- nothing 

 more than a gradual coale-cence of the ventral curves of the inner an<l outer ri< 

 ami the con-e.|uent ob- re of the -ulci. 



That /.'. tri*nl<:iln ami similar form- could not have been developed from Un- 

 typical tnlol-e.l (hisul' iri'-hin, nor from l\l,nleni<i is perfectly i-lear t<> me. 



Miice if wniilil I v to assume a division of the small or middle lol.e of t ! 



which I think I am safe in declaring, never took place. 

 i'lel/n is essentially a Lower Silurian gemis. nearly all the typical |p< 

 I.ein^ re-tricted to strata lielon-ini.' to. or e-|iiivalent to the Trenton and Cincinnati 

 formation-. In America we have '/'. ./i/WnV/'/v/A; Hall and Whit Held, and var. *///, / 

 I Irich. 7'. lun.ifiirrd and 7'. suln/inKlnil'i Uriel). Of Kiiropeaii species donl.ti. 

 lu'lon^in^ to '/'. ',--i. l,-ll'i I may mention /.V//n'<7j/,/ roin/iHi-.il'i Salter, />'. rilii-iriunn 

 lone-, li. iijfini.-< .].. II. l>uxii<i . /'. l<ininnt>i .1.. /'. ni<irrlii,;i l\rau-e. /.'. rrr>itii;i K.. 



/;. /..//MM/.I K . / K. /'. '!'/'/ K . a:i'l T. ii'ir/." K. As somewhat .h.iil.tfnl 



l'[iper Silurian representatives, we m ,rd four species figured ly I >r. Krause, 



vi/..: It, >/,-!. l<i. i ,li,iil.il<t K.. /'. duMOfa K . /.'. in-imil/im-i K.. and //. nixliilnsn Holl. In 

 the first the ridges do not ai'p.-arto unite ventrally. and in the last the anterior pair 

 are peculiarly twisted together, while in the second and third all the ridges are 

 divided into m-. nodes taking the place of each ridge. 



i AIIRII.IKATA Hall and 1 17/// //,//. and varieties. 



i-i.ATi: M.VI mis i ii. 



BeynV/,1.1 ,,u,i,lrili:<il,, II. :r >llio, Vol. 



I fl. regulori* Km 

 ). ii. I ;i Cmilri. 

 , .;u.i./ri/ir.i/.i I'l.liicil. I "."i. .I-) 



. I. Hi nun : tii. 'inn. 



J IMIM.: thickness <).;t* HUM. 



l.i:i tu HI .: liiK'tit 0.70 IMIM.: tliii-knvmafiS nun. 



Figures 1 t. from a represent;!' inn-n of the specie- a- it 



occurs in the Trenton shale- of Mimie-ota. I' y nearly identical with the 



typical form which i- found so abundantly in the up| f the Cincinnati group 



in Ohio and Indiana. The original of tig. t i- from the ISir-l-eye limestone at High 



Micky. Thi- i- -omewhat shorter and more ohli.jue than u-iial. Figur.- 



a variety, not uncommon at. Minneapolis, in which the aiitero-incdian 



