MEBISTELLA OF THE HAMILTON GROUP. 
805 
tened below the middle, becoming depressed towards the front, which 
in old shells is produced into a short linguiform extension : umbo 
gibbous, the beak arching over the umbo of the opposite valve and not 
closely appressed. 
Dorsal valve little longer than wide, varying from moderately convex 
to gibbous, the greatest convexity being about the middle of its length ; 
without distinct mesial fold, but abruptly elevated near the anterior 
margin, corresponding to the depression on the opposite side. 
Surface smooth, or marked by regular concentric strise which are some¬ 
times crowded into wrinkles near the margins of the valves. The 
exfoliated shells sometimes show indistinct radiating strise. 
The muscular impression in the ventral valve is triangular, and usually 
not deeply marked. The dorsal valve has a distinct median septum which 
extends nearly half the length of the valve : muscular area narrow, 
elongate. 
This species presents considerable variety of form, from almost sym¬ 
metrically oval to broadly ovate with the greatest width below. The older 
shells are for the most part gibbous, but some specimens are compressed in 
the lower half of the length. The mesial sinus is not usually a very dis¬ 
tinctive feature in half-grown shells; but in some individuals it begins 
about the upper third of the shell, and affects the lower half and ante¬ 
rior part of the valve. 
The largest individuals have a length of about one inch and a quarter, 
with a width of one inch; in other specimens, a length of one inch and 
an eighth gives a width of one inch and a depth of three-fourths of an 
inch. In a gibbous and somewhat elongate form, the depth and width 
are as 5 to 6, and the length 8j. 
Figures 5-12, and 16 & 17, represent the most characteristic forms. 
Figures 13 & 14 are of an extreme form. 
Figure 20 is one of the broader forms. 
Figures 21 & 22, dorsal and front views of a cast of this species. 
Geological formations and localities. This species has been found chiefly in a 
limestone of the Marcellas shade, near Le Roy, N. Y. It occurs in the Hamilton 
group, near York in Livingston county, New-York. 
[ Palaeontology IV.] 39 
