Lower Silurian Fauna of Minnesota. 337 



METOPTOMA EXPLANATA n. Sp. 



Plate VI, figures 7 and 8. 

 Shell oval, broadest anterior to the middle, low, very rapidly ex- 

 panding from the apex to the aperture, apex small, acute, nearly 

 straight, and directed posteriorly. Anterior slope long, 18 mm., pos- 

 terior slope short, 5 mm. Shell thin and with no surface ornamenta- 

 tion. The specimen figured is from the Blue limestone at Minneapolis. 

 This species very closely resembles M. perovalis Whitf., from the same 

 horizon at Beloit, Wisconsin, but on the two specimens in hand no 

 truncation of the posterior margin is apparent. 



PLEUROTOMARIA CLIVOSA n. Sp. 



Plate VI, figure 9. 



Compare R. spousa Billings. 



Shell small, conical, apical angle 75 degrees; length 12 mm., breadth 

 10 mm. ; consists of about five volutions turreted one above the other. Cari- 

 nated; one carina marks the limit of the umbilicus and is obtusely 

 rounded; a second, below the middle on the outer volution and concealed 

 below suture line in the first volutions, is acutely rounded; a third carina 

 is on the outer extremity of each whorl and is angular. From the suture 

 above each volution the surface rounds out broadly but becomes concave 

 towards the main carina; the narrow band between this and the next 

 carina (or the suture) is concave; from the last named to the umbilical 

 carina the surface, in general outline, is convex, but is slightly undulated 

 making two broad indistinct carina. Transversely marked by growth 

 lines which curve obliquely back from the suture to the main carina and 

 thence undulate obliquely forward to the umbilicus. Umbilicus small. 

 Aperture vertically subelliptical, obliquely subquadrate, with the lower 

 outer wall of the penultimate volution forming the upper inner boundary 

 of the aperture. Lip (?). Shell thick. From the Stictopora bed at Min- 

 neapolis. 



TRYBLIDIUM EXSERTUM n. Sp. 



Plate VI, figures 3 and 4. 

 Shell high and curved, forming about one-third of a volution; grad- 

 ually expanding to near the aperture which (as shown on an interior cast) 

 is suddenly expanded. The interior cast is marked by fine radiating lines 

 two or more to i mm. These are shown on casts of the exterior to have 

 been narrow acute bifurcating elevations, also marked by less acute and 

 finer concentric lines. Aperture oval, much narrowed under the beak and 

 with an unbroken margin. Length of the aperture 30 mm., breadth 25 

 mm. The muscle scars are not distinct on the cast, but there appears to 

 be a row of about 24 passing around the shell from 2 to 5 mm. above the 

 lip of the aperture. From the (2d) Blue limestone at Minneapolis. 



TRYBLIDIUM VALIDUM n. Sp. 



Plate VI, figures i and 2. 

 Interior cast elliptical in outline, subtruncate posteriorly ; somewhat 

 acutely rounded anteriorly. Length and breadth of the aperture and the 



