GENUS LEIORHYNCHUS. 
U55 
Genus Leiorhynchus (Hall). 
Leiorhynchus : Hall, Thirteenth Report on the State Cabinet, p. 75. 1860. 
The shells of this genus are ovate, circular or transverse, with valves 
unequally convex, and marked by a median sinus and fold in the ven¬ 
tral and dorsal valves respectively. 
The surface is plicated by rounded bifurcating plications which are 
always more conspicuous on the mesial fold and sinus, while they 
often become obsolete on the lateral portions of the shell; concen¬ 
trically marked by strong lines of growth. Substance of the shell 
fibrous, usually thin. 
V alves articulating by teeth and sockets; the apex of the ventral valve 
perforate at some period of its growth, the lower side being completed 
by deltidial plates. On the interior of the ventral valve, two short 
diverging dental lamellae extend into and are joined to the sides or 
bottom of the rostral cavity; the muscular impression occupies a 
narrow triangular or ovate-triangular space below the dental lamellae. 
The dorsal valve has a well defined septum, often reaching below the 
middle of the valve, and divided above, leaving a triangular or spoon¬ 
shaped depression. The hinge-plates are narrow strong processes, with 
sockets embracing the teeth of the opposite valve. 
Numerous examples, showing the interior to some extent, have not 
developed any spiral appendages in the shells of this genus, and I am 
now quite satisfied that they belong to the Rhynchonellidae. Externally 
these shells differ from Stenocisma in the low rounded and bifurcating 
plications, and their obsolescence on the lateral margins, while they are 
never interlocked at the junction of the valves. When occurring in the 
same beds, the two have a different aspect; and Leiorhynchus is abun¬ 
dant in the black shales where Stenocisma rarely occurs. It appears, 
therefore, desirable to continue these forms under a distinct genus. 
These shells are at present known only in the Hamilton and Chemung 
groups; beginning their existence in the Marcellus shale, being quite 
abundant in some dark shaly beds in the middle of the group and in the 
