Genus Leiorhynchus. Han. 



Leiorhynchus, Hall. 13th Regent's Report 1860. 

 Etymology : leios, smooth ; rhynchos, a beak. 



Shell ovate, circular or transverse, with valves unequally convex, and marked 

 by mesial sinus and fold in the ventral and dorsal valves respectively. The 

 surface is plicated by rounded, bifurcating ribs, which are always more con- 

 spicuous on the mesial fold and sinus, whilst they often become obsolete on the 

 lateral portion of the shell ; concentrically marked by strong lines of growth. 

 Substance of the shell fibrous, usually thin. Valves articulated by teeth and 

 sockets ; the apex of the ventral valve perforate at some period of its growth, 

 the lower side being closed by deltidial plates. On the interior of the ventral 

 valve, two short diverging dental lamellae extend into and are joining 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. (Hall.) This genus is closely allied to that of Rhyn- 

 chonella. 



Leiorhynchus quadricostatus. VANVXKM. 



' Orthis quadricostata, Vanuxem. Geol. Rep. 3d dist. N. Y., p. 186 1842. 

 Orthis quadricostata, Hall. Geol. Rep. 4th dist. N. Y., p. 2231843. 

 Leiorhynchus quadricostata, Hall. 13th Rep on St. Cab., p. 86 1860. 

 Leiorhynchus quadricostata, Hall. Pal. N. Y.,.Vol. IV., p. 3571867. 



Shell of medium size ; broadly ovate ; from moderately convex to somewhat 

 gibbous ; with distinct mesial fold and sinus. 



Ventral valve a little gibbous towards the beak; side nearly flat, with a 

 mesial sinus commencing below the umbo and extending to the front ; it in- 

 creases gradually in width and depth towards front ; umbo is of medium size, 

 and beak elevated and pointed, and slightly arched but not incurved ; place of 

 the apex appears to be occupied by a round perforation. Dorsal valve is more 

 convex than the opposite, having its greatest convexity in the middle of the 

 valve, from where it slopes in a light curve to the baso-lateral margins, and in 

 a stronger curve to the beak and cardinal margins. A mesial fold commences 

 to rise above the surface in front of the umbo, from where it slightly increases 

 in height and width towards the front. The dorsal umbo is low, beak small 

 and incurved. The surface is partly plicated. On the mesial elevation and in 

 the mesial sinus are generally 'four plications plainly visible in the basal half. 



