610 THE PALEONTOLOGY OF MINNESOTA. 
[Lyrodesma cannonense, 
not, as isthe casein Minnesota, there associated with the typical form. The variety, 
~ which may take the name of intermedium, connects L. acuminatum with L. cincinnatiense 
Hall, being as nearly as possible intermediate between these species. The form of 
the shell and the prominence of the umbonal ridge will distinguish L. acuwminatum 
from all of the other species. 
Formation and locality —Middle third of the Trenton shales, Chatfield and near Cannon Falls, 
Minnesota. The var. intermediwm occurs at the same localities and in the Trenton limestone near 
Burgin, Kentucky. 
LYRODESMA CANNONENSE, 7. Sp. 
PLATE XLII, FIGS. 6—8. 
Nucula poststriata HALL, 1847. Pal. N. Y., vol. 1, p. 151, pl. 34, fig. 2a, 2b. (Not p. 301, pl. 82, 
figs. 10a, b.) ‘ 
This small species of which only casts of the interior have been seen, is similar 
in shape to L. acuminatum var. intermedium. Critically compared it proves to be 
longer, and has the beaks farther anterior. The hinge line also appears to have been 
somewhat longer. Then there is a slight depression on the posterior side of the 
umbones which is not seen on casts of that species. In all these respects L. cannon- 
ense agrees very closely with L. subplanum, a new species from the Utica horizon of 
the Cincinnati group, at Covington, Kentucky, which I am describing in vol. vii of 
the reports of the Geological Survey of Ohio; and it is with that form that I believe 
its relations really lie. Comparing it with an excellent cast of that shell, the 
Minnesota form is distinguished by its shorter hinge line, more oblique posterior 
margin, more sharply angular umbonal ridge, and wider beaks. Though also smaller 
it cannot be denied that the two forms are very closely related, and probably nothing 
more than varieties of one species. 
The Trenton shell referred to by Hall in 1847 (Joc. cit.) to Nuculites (now Lyro- 
desma) potstriatum Emmons, is not the same as the Hudson River type of that 
species, but probably belongs to L. cannonense. 
Formation and locality.—Galena shales near Cannon Falls, Minnesota. Also in the Trenton lime- 
stone, Carlisle, Pennsylvania. 
