GENUS LEIORHTNCHUS. 253 



GE\IJS LEIORHYNCIILS (Hall). 



Leiorhtnchus : Eai-l, Thirteenth Report on the State Cabinet, p. 76. 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. 

 Valves 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 lamellaB. 

 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 Rhynchonellidas. 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 



