39 



Description. — Shell sub-ovate, apical angle from 90^ in the smaller 

 specimens to 130^ in the larger ; greatest width about the mid-length, or 

 a little in front thereof ; slope on each side of the beak straight or slight- 

 ly concave ; sides rounded ; front margin with a small portion in the mid- 

 dle about equal to one-fourth or one-third the greatest width of the shell, 

 varying from narrowly rounded to straight. On each side of this median 

 portion the margin is nearly straight and converging from near the mid- 

 length towards the front at an angle of about 45^ to the longitudinal 

 median hne of the shell. Ventral valve strongly and uniformly arched, 

 from the beak to the median point of the front margin, most elevated 

 about the mid-length, broadly and moderately convex in the posterior 

 half or two-thirds ; mesial sinus about one-third the whole width ; the 

 bottom either flat or slightly convex in the middle, strongly elevating 

 the front margin of the dorsal valve; becoming obsolete at the mid-length 

 of the shell or a little above. The shell on each side slopes down into the 

 sinus with a convex curve. Beak closely incurved down to the umbo of 

 the dorsal valve. Dorsal valve strongly convex, its greatest depth about 

 the mid-length or a little in front thereof, posterior half broadly and uni- 

 formly convex ; mesial fold strongly elevated at the front margin, flat or 

 gently convex on the top, becoming obsolete at one-third or one-half the 

 length, the sides (of the fold) sloping down to the body of the shell with 

 a flat or sometimes gently convex slope. 



On a side view, (fig 20.) the line of junction of the margins of the two 

 valves forms a deep curve from near the beak to the front, convex 

 towards the ventral valve. In the dorsal valve there is a median septum 

 which extends from the beak nearly one-half the length of the shell. 



Surface with numerous small sub-angular radiating ribs, usually three 

 in the width of two lines at the margin in the front half, becoming finer in 

 the upper half. In some specimens these are obscurely crenulated by 

 concentric lines with occasional stronger rugae of growth. The ribs on 

 'each side of the fold and sinus are generally bifurcated, the others (on 

 the fold and sinus) are more often simple or undivided. 



Length of a nearly perfect specimen, seventeen lines ; width, fifteen 

 lines ; depth of both valves, eleven lines. 



Locality and Formation. — Indian Cove, Gasp^, in the Gasp^ lime- 

 stone. No. 8. 



Collector.— K Bell. 



