661 



*t*-J 



inner side of the \alve. we see nothing of the outer wall of the marginal area. The 

 I regard a- the mo-t important diM'ei> if tin- fact- an- correctly repre- 



,-enteil in 1'rof. .lime-' figures it would remove lii- from the typical section of 



I\tiri/rliilin<i to that di-tinct group of spec in- which i- delined on a preceding page 

 in the remark- following tl i ic de-cription. 



Variety Q n. var. 1'l.ite \i iv. : 



Thi- siiliordinate name i- propo-ed fora variety of this species that is rarely 

 associated with more typical specimen- in the upper third of the Trenton shales, the 

 highest, hori/.on in which tin- -pecie- i- known to occur. The variety is a little 

 smaller than full grown -pecimen- of the typical form, and more rounded in the 

 po-terior iiitlme. Mure -hiking ditlerences however are seen in the marginal 

 area. This, in-tead of being conca\e and curved outward, is convex and incurved, 

 its width is less and more eo.ual. the radii very indistinct ami the terminal border 

 more sharply delined. These dill'erences produce a form closely resembling the 

 Kentucky specie- A'. <j<". \\ ',- except of course the ornamentation, the two being 



nt in this respect. 



ilily Range* fpiiu tin- lnwi-r Trvntoii liriif.-torir in HP- II|I|T third of the Tren- 



Paul, Caiitiiin Falls, and n.'.ir Fountain. Minnesota. The specie* Is u..| 



abundant any u . aUnit twi-nty spc.-iiii.-n- in all having been &>n. ' ..-urru ocrurs an far 



Kl K-iCIIII ISA -IHKAD1ATA I'lri'h. 

 PLATE XI.IS : H 



//. Ai/i.,,1 u i,r,i>lial,i L'LKK n, 1890. Jour. Cln. Soc. Nat. Hist., vol. xill, p. 128 



it marginal ar .-tit " ' nun.: Ihii-knt-s-s 0.50 mm 



With tnar^'ir j nun.: hiL-Itt l..'t mm. 



1'iodv of valves almo-t exactly -I'micirciihir in outline, with the -urface highest 

 along an olitu.-e ridge-like prominence, running lengthwi.-e across the central portion 

 of the valve and from the summit of which the .-urface descends with a distinctly 

 concave slope to the thickened dor-al ed^e; on the opposite or ventral side the slope 

 is more gently concave or Hat: anterior extremity compre ed; sulcus deep and 

 unusually wide. l.ei:iiining a little within the dor-al margin and extending half way . 



M the body, it< lower and i margin- thickened and -harply defined: ju-t 



hack of the -ulcu- a large round tul-ercle; -urface appearing Miiootli in -ome >| 

 mens. l-ut usually it is pitted a- <hown in (Lv 1. Marginal area nearly Hat. the inner 

 edge rising abruptly and forming a low. sloping wall around the body, the outer 

 edge formed by a sharply elevated narrow bonier; posterior and ventral portion- of 

 area holding about the same width, but at the anterior end it i- u-ually much leas; 

 external -urface of area with more or less obscure radial furrow-. Inner -ide of 



