10 PEOCEEDINGS OF THE NATIONAL MUSEUM vol.69 



two niiddle ones joining to form a V. Rest periods about seven 

 indicated by stronger grooves than those of the minor concentric 

 sculpture and by a concentric darkening of color. Color chestnut 

 brown, the posterior half darker than the anterior. Periostracum 

 glossy, paperlike, tending to peel at the posterior margin. Prismatic 

 layer very thin, about the same thickness as the periostracum and 

 peeling with it. Nacre silvery white, slightly iridescent at the pos- 

 terior portion, an indistinct groove from the beak to the ventro- 

 posterior corner. Anterior adductor scars deep, posterior scars 

 superficial. Pallial line indistinct, about 10 mm. from the ventral 

 margin. Prismatic margin very narrow. Pseudocardinal teeth 

 rather thin, platelike, set nearly parallel to the dorsal margin. In 

 the right valve the outer tooth is the smaller while in the left valve 

 it is the larger. Just under the beak in each valve is a third portion 

 of the pseudocardinal, small and set transversely to the hinge line. 

 The lateral teeth are somewhat bowed, the single lateral of the right 

 valve being very high and thin. Of the two laterals in the left valve 

 the inner is the larger and the groove between the two teeth is deep 

 and narrow. All the lateral teeth are crenulate and much striated. 

 The type, Cat. No. 363027, U.S.N.M., measures : Length Y6 mm. ; 

 height, 52 mm. ; diameter 26 mm. It comes from Tigre River, Tigre, 

 Buenos Aires, Argentina. Cat. No. 348794, U.S.N.M., includes two 

 younger specimens from the same place. All three specimens were 

 collected and presented by Dr. Hugh M. Smith, for whom the species 

 is named. 



ANODONTITES IRISANS, new species 

 Plate 2, figs. 3. 5 ; plate 3, fig. 7 



Shell elongate-oblong, compressed, rather thick, rounded and nar- 

 rower in front, truncately rounded, wider and somewhat nasute be- 

 hind; dorsal margin lightly arcuate, making an indistinct, very 

 obtuse angle with the posterior margin and rounding into the an- 

 terior margin. Ventral margin nearly straight, sloping gently up- 

 ward toward and fading into the anterior margin without any angle. 

 Anterior area descending rapidly from the body of the shell without 

 a ridge ; posterior area gently sloping, the posterior dorsal ridge low 

 and rounded. Posterior dorsal area with a faint rib running from 

 the beak to just above the posterior ventral angle. Surface nearly 

 smooth but with many fine concentric growth lines and a few heavier 

 lines marking rest periods of growth. Anteriorly and posteriorly 

 the fine growth lines are more prominent, forming fine lamellae. To 

 the unaided eye there are faint indications of radiating striae. With 

 a fairly high power microscope the periostracum shows innumerable 

 radiating threads. Color chestnut brown throughout. Interior 

 brilliantly pearly, very iridescent along the anterior, posterior, and 



