248 



Streptorliyiichus Agassizii, Uartt, sp. iiov., Plate IX, figs. 3, 4, 10, IG, 17, 



23, 25, 26 and 28-30. 



Test small or of moderate size, never very large, transverse, double convex, 

 or with the ventral valve sometimes slightly and irregularly flattened, or even 

 concave towards the front. Valves subequally convex, varying much in out- 

 line ; sometimes unsymmetrical ; often circular, with the sides and front forming 

 a segment of a more or less perfect circle, embracing two-thirds to three- 

 fourths the diameter ; in some cases sub-elliptical ; seldom transversely ob- 

 long. Greatest width at or a little behind the middle. Cardinal extremities 

 rounded or obtusely angular, but in a single known instance slightly extended 

 into mucronate points. Proportions of length to width about as 2 to 3, 3 to 4, 

 or 3 to 5. Hinge line straight, beak of ventral valve elevated and pointed, that of 

 dorsal valve being depressed and broadened. In size, the test varies from very 

 young and small to 21 m.m. long, by 32 m. m. broad, and one unusual speci- 

 men measures 29 m.m. long by 40 m.m. broad. Surface ornamented with 

 very fine raised lines. 



Ventral valve most prominent at, and in the neighborhood of the beak. Ex- 

 tremity of beak generally elevated above the rest of the valve, with the 

 surface of the valve sloping more or less irregularly towards the sides 

 and front, and either straight, slightly concave or somewhat convex; or 

 the beak may be a little depressed, the surface rising from it for a short dis- 

 tance, and then continuing to the front and sides as in the former case. In a 

 few exceptional instances, the valve is regularly and strongly convex, from the 

 beak nearly to the front ; but the general tendency in all specimens is, for the 

 surface to flatten out toward the front and sides. Beak acute, seldom per- 

 fectly symmetrical, generally bent a little to one side or the other, or slightly 

 twisted ; never extending much beyond the hinge area, and incurving but little. 

 Hinge area nearly or quite as long as the hinge line, of moderate width, trian- 

 angular, inclined backward, and with the cardinal margins acutely angular. 

 The area is never symmetrical in outline, and its form varies as the beak is 

 bent or twisted. The cardinal margins are generally concave, though they 

 vary in degree of curvature in the same specimen, and may be slightly con- 

 vex on one side of the beak, and concave on the other. In extending towards 

 the cardinal extremities, they generally bend more or less abruptly towards 

 the hinge line, sometimes approaching quite near to it at some distance from 

 the extremities, so that the hinge area may be very narrow at the sides, and 

 appear as if quite short. The surface of the valve sometimes arches up quite 

 rapidly on the sides, from the cardinal margins, for a short distance. Fissure 

 of moderate size, triangular, the width at base equal to or slightly exceeding 

 the height, covered by a very convex deltidium. The impressions of the den- 

 tal plates in the internal moulds are visible only on the hinge area, at the 

 sides of the fissure, appearing as shallow depressions not extending forward 

 into the valve. 



