82 
BULLETIN OF THE LABORATORIES 
have seen, belong to a smaller type of the var. lynx^ and are charac- 
terized, like our Clinton specimens, by an even number of plications 
in the sinus, and odd number on the mesial fold. 
V. Forma Daytonefisis. 
{Plate XIII, Fig. 8.) 
This is another form belonging to the forms typified by the varie- 
ties of O. biforata, found in the Cincinnati formations. They bear 
considerable resemblance to the young of var. lynx, and in one case, hav- 
ing the hinge line greater than the breadth of the shell, it varies somewhat 
in the direction of var. acutilPata. The name is expected to have 
only a local use for the Clinton forms which have two plications in the 
mesial sinus, and three simple ones on the fold, all of which seem to 
originate at about the same distance from the beak. 
Shell of small’ size, wider than long, with a subquadrate outline, 
hinge line usually shorter than or equaling the breadth of the valve, in 
one specimen exceeding it in size. The shells have their outline indis- 
tinctly preserved and are found only as single valves showing their 
exteror surface. 
Ventral and dorsal valves with sinus and fold rounded, the plica- 
tions also more rounded than those of Lower Silurian formations, 
simple. 
These specimens could perhaps be considered as the young of the 
form reversata, werCit not for the simple plications on both fold and 
sinus, which remain simple where on the corresponding places of the 
other form there would be several additional plications intercalated. 
Length of medium sized specimen, 12 mm.; breadth, 18 mm.; 
convexity, as well as can be determined by a comparison of different 
single valves, 9 mm. Breadth of largest specimen, 24 mm. (The 
name of the form is taken from the locality where it is most frequent. 
locality and position. Soldiers’ Home, Clinton Group. 
VI. Orthis flabella, Soiverby. 
[Plate XIII, Figs. i3 a, b.) 
Shell semi-oval ; hinge line equal to the breadth of the valve or 
generally a little less. Shell wider than long, the dorsal valve much 
more convex than the other, very variable. 
