﻿HYRIOPSIS 20Q 



Hyriopsis gractijs (Haas). 



"Shell long, narrow, alate. rather solid. Anterior end very 

 short; the superior margin somewhat curved in front and 

 drawn out in a horizontal or bent up point, meeting the anterior 

 margin at a distinctly sharp angle; anterior margin concave 

 above, slightly convex below as it passes into the almost hori- 

 zontal, slightly convex basal margin; posterior end rounded 

 and slightly biangulated ; posterior margin oblique ; wing near- 

 ly one-half the height of the shell, concave behind, rounded 

 at the end and nearly straight in front, meeting the dorsal 

 margin at an angle of 45^ a short distance behind the beaks. 

 Beaks anterior, placed at 19/100 of the total length, not pro- 

 jecting nor swollen, sculptured with a few, concentric, wavy 

 wrinkles. From the beaks to the biangulations of the posterior 

 end extend two feeble, thread-like lines, which become lower 

 posteriorly. Ligament strong, covered by the wing. Cardinal 

 teeth weak; two in the right valve, the inner one lower and 

 lamelliform, the outer one subobsolete, lamelliform; in the left 

 valve a rather stouter, lamelliform tooth, in front of two feeble 

 ridge-like teeth ; lateral teeth long, straight, two in the left and 

 one in the right valve. Angle of the cardinal teeth 40° ; angle 

 of the lateral teeth o'. Nacre bluish-white. Muscle impres- 

 sions weak. Epidermis black-brown or brown-green, eroded 

 on the beaks. 



Length no, greatest height 68, height at the beaks 39, height 

 of the wing 27, diam. 19 mm." (Haas.) 



Type locality, Bienho-Sei, Mekong. 

 Hyriopsis gracilis Haas, Nachr. Deutsch. Mai. Ges., 19 10, p. 



lOI. 



Group of Hyriopsis cumin gii. 



Shell rather thin, with a high wing posteriorly and a smaller 

 one in front ; beaks greatly compressed, their sculpture at first 

 consisting of fine and later of coarse concentric ridges, which 

 extend well on to the disk ; corrugations on the posterior slope 

 distinct. 



