346 THE PALEONTOLOGY OF MINNESOTA. 



[Llngula cobourgensis. 



the others. In several species of recent Discinisca such as D. strigata Broderip and 

 D. Icevis Sowerby the same kind of transverse lines are usually present on the ventral 

 valve, orginating on each side of the pedicle slit, curving outwardly and terminate 

 on the anterior third of the shell. In the above species of Lingula and Lingulella it 

 is a constant feature in both valves, while in Discinisca it may or may not be devel- 

 oped in the same species and when present is restricted to the ventral valve. 



Prof. Whitfield remarks that " the peculiar surface ornamentation of Lingulella 

 stoneana seems to be one that characterizes many forms of this genus in the prim- 

 ordial formations." Since Lingula is a development from Lingulella the presence of 

 the transverse lines in species of the former genus seems to indicate that they have 

 acquired it through heredity rather than its being a sporadic development. If this 

 inference is the correct one it seems to call for some recognition in a future classifi- 

 cation of species of Lingula. 



Prof. Whitfield also calls attention to a homologous growth in Lucina divaricata 

 and other pelecypods and thinks this feature to depend " on a series of changes in 

 the mantle of the animal during the formation of the shell." While we do not 

 doubt this growth being due to a deposition from the mantle, yet how shell can be 

 secreted externally, posterior to the anterior edge by a "series of changes in the 

 mantle " is not clear. 



Formation and locality, A number of specimens were found by Mr. Ulrich in the middle third of 

 the Trenton shales at St. Paul associated with Rhinidictya mutabilis. 



LINGULA COBOURGENSIS Billings? 



PLATE XXIX, FIG. 12. 



1862. Lingula cobourgensis BILLINGS. Palaeozoic Fossils, vol. i. p. 50, flg. 54. 



1863. Lingula cobourgengis BILLINGS. Geology of Canada, p. 161, flg. 132. 



Original description: "Almost regularly oval; greatest width about the middle; 

 length one-fourth greater than the width; anterior extremity uniformly rounded; 

 apex obtusely angular; both extremities sub-equal; sides gently convex. Both 

 valves are moderately convex, and one of them has sometimes an irregular furrow 

 extending from near the beak along the median line for one-half or three-fourths 

 the length. Color dark brown, with some shades of light brown or yellow; general 

 aspect smooth and shining, with fine concentric undulations of growth, which become 

 fine, elevated, sharp, closely crowded strias, on each side; longitudinal striae are visible 

 on some specimens. 



"Length, about one inch; width, about three-fourths of an inch; depth of both 

 valves, three or four lines. Smaller specimens occur associated with the larger." 



