610 THE PALEONTOLOGY OF MINNESOTA. 



[Lyrodesma oannoneuse. 



not, as is the case in 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. acuminatum 

 from all of the other species. 



Formation and locality.— MidAle third of the Trenton shales, Cbatfleld and near Cannon Falls, 

 Minnesota. The var. intermedium occurs at the same localities and in the Trenton limestone near 

 Burgin, Kentuclfy. 



Ltbodbsma oannonense, n. sp. 



PLATE XLII, FIGS. 6-8. 



Nucula poststriata HALL, 1847, Pal. N. Y., vol. 1, p. 151, pi. 34, flg. 2a, 2b. (Not p. 301, pi. 82, 



flgs. 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. suhplanum, 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 (loc. cit.) to NucuUtes (now Lyro- 

 desma) podstriatum Emmons, is not the same as the Hudson River type of that 

 species, but probably belongs to L. cannonense. 



Formation and loeality. —Galewd shales near Cannon Falls, Minnesota. Also in the Trenton lime- 

 stone, Carlisle, Pennsylvania. 



