52 GEOLOGICAL AND NATURAL HISTORY SURVEY OF CANADA. 



I doubt veiy much that this is a true Ptimitia, but, as it seems more 

 nearly related to some of the forms placed there by Prof. Jonos than 

 to the species of any other genus known to me, I thought it best to 

 describe the species as above. It may be compared with some of the 

 more simple forms of Beyrichia. 



EURYCHILINA. (N. gCn.) 



Valves semicircular, suboval, or even nearly circular. Dorsal line 

 straight. Generally with a well-defined sub-central sulcus and a more 

 or less prominent node just behind it. A very broad convex bordei* 

 extends around the valves from the antero-dorsal to the postero-dorsal 

 angle. The border is often striated in a radial manner and, in rriost 

 cases, terminated by a marginal " frill," or by a plain narrow border, 

 usually directed slightly outward. The main body of the border, how- 

 ever, curves inward to near the plane of contact between the two 

 valves, thus forming a deeply concave outer ai*ea. Hingement simple. 

 Surface reticulate, granulose, or smooth. 



Type: E. reticulata, n. sp.. from the Trenton shales of Minnesota. 



This genus is related to Primitia, Jones and Holl, and to Primitiopsis, 

 Jones. The possession of an internal concave marginal area separates 

 it from the first, while the greater extension of the hollow area distin- 

 guishes it from the second. In Primitiopsis the mai-ginal hollow is 

 developed only at ihe anterior end. 



Ten species and several varieties having the characters ascribed to 

 the genus are known to me from Amei'ican Cambro-Silurian deposits. 

 Two of these are from the Chazy, three from the Birdseye, three from 

 the Trenton, and two from the upper beds of the Hudson River or 

 Cincinnati group. One of the last has been described by Mr S. A. 

 Miller, under the name of Beyrichia striatomarginata. From sketches 

 just received from Prof. Jones, it appears that Primitia strangula/a, 

 Salter, sp., should also be referred to Eurychilina. 



EURYCHILINA RETICULATA. (N. Sp.) 



PI. IX., figs. 9, and 9a. 



Yalves nearly semicircular in outline, the curve of the anterior end 

 being slightly less i-apid than that of the posterior, and the height from 

 the ventral edge to the straight dorsal margin a little greater than it 

 would be if of true semicircular shape. Body of valves moderately 

 convex, with the point of greatest convexity sub-central. Sulcus deep, 

 beginning a little below the dorsal mai'gin and extending half way 

 across the body ; its lower and posterior margins somewhat thickened 



