CAMBRO-SILURIAN MICRO-PAL .EONTOLOGY. 53 



and sharply defined. Sui-face, except along the dorsal line, beautifully 

 reticulate. Marginal area wide, its width equal on the anterior and 

 ventj-ni sides : as it rounds up on ihe posterior end its breadth gradually 

 diminishes till at the postero-dorsal angle it is scarcely more than one- 

 fourth as wide as the antero-dorsal angle. As the area leaves the body 

 of the valve it is convex, but soon the development of a radially marked 

 '' frill" makes it slightly concave. The " frill" is free only at its outer 

 extremity, and this may be dentate, entire or terminated with a 

 thickened rim, the variations probably following each other with age. 

 In some examples the line of junction between the body of the valve 

 and the marginal area is marked by a more or less strongly elevated 

 thin ridge. In others there is simply an impressed suture line. The 

 radial lines are strongest along the ventral edge and least distinct at 

 the anterior end. 



The interior shows that the width of the marginal hollow is equal to 

 nearly one-foui-th of the height of the entire valve, and that its outer 

 <3dge curves inward nearly to the plane of contact between the two 

 valves. The two sections shown in fig. 9a are faulty in representing 

 the outer and lower extremity of the marginal areas as being simply 

 thickened. The lower ends of the two sections ought really to have 

 been represented by two diverging lines, with the inner one also more 

 extended inward than now appears. 



The dimensions of an average specimen are as follows : Greatest 

 length of valve, including marginal area, 2.5 mm.; length of dorsal 

 line, 2.3 mm. ; greatest height of valve (across the posterior third), 

 1.4 mm.; length of body of valve, 1.8 mm.; greatest width of same, 

 1.0 mm. ; greatest convexity of same (one valve), 0.25 mm. ; width of 

 mai'ginal area at the anterior end, 0.4^ mm. : width of same at the 

 postero-dorsal angle, 0*12 mm. ; width of ventral portion of marginal 

 hollow (inner side of valve), 0.3 mm. 



This species is distinguished from all the others known by its neatly 

 reticulated surface. Separated valves are not uncommon in the Tren- 

 ton shales at Minneapolis, St. Paul, Fountain and other localities in 

 Minnesota. It is described in this connection because it is better 

 adapted to stand as the type of the new genus, Eurychilina, than any 

 of the others. 



EURYCHILINA MaNITOBENSIS. (N. Sp.) 



PI. IX., figs. 10, and 10a. 



Yalves smooth, semicircular in outline, the body very little convex, 

 its two ends almost equal and forming nearly right angles with the 

 straight dorsal line. Yentral margins of bod}^ and marginal area 



