346 THE PALEONTOLOGY OF MINNESOTA. 
[Lingula 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 Lingiilella 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. Limjula cobourgensis Billings. Palaeozoic Fossils, vol. i, p. 50, fig. 54. 
1863. Lingula cobourgensis Billtnos. Geology of Canada, p. 161. flg. l.'}2. 
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 iiTegular 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 striie, 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." 
