1897.] NATURAL SCIENCES OF PHILADELPHIA. 293 



Length 20 - 5, height 13'8, diam. 7 - 5 mm. 

 Length 20, height 14 mill. 

 Maldonado Bay, Uruguay. 



T. fragilis Penn. is longer in proportion to the height, more pro- 

 longed and narrower posteriorly, and more convex. 



Seraele (Abra ?) Uruguayensis n. sp. PI. VII, figs. 27, 28, 29. 



Shell thin, inequilateral, the anterior end conspicuously longer, 

 convex, smooth except for fine, faint growth-striae. Surface slightly 

 glossy or dull, pale isabelline, becoming white toward the beaks. 

 Dorsal margins sloping abruptly each side of the beaks, the ante- 

 rior slope nearly straight, posterior slope slightly convex ; anterior 

 end broadly rounded, posterior end rather narrowly rounded below ; 

 basal margin regularly rounded, becoming a little straighter near 

 the posterior end. Beaks small, slightly projecting ; a faint ridge 

 extending from them to the junction of the posterior and basal 

 margins. Right valve with a small, erect and vertical posterior 

 cardinal, and longer, larger oblique anterior cardinal tooth, and a 

 low, slight, lamellar anterior lateral ; no posterior lateral tooth. 

 Left valve with an erect vertical cardinal tooth, no laterals. Inte- 

 rior pure white ; pallial sinus very large and deep. Length 9'5, 

 height nearly 8, diam. 4'5 mm. 



Maldonado Bay, Uruguay, in 3 to 6 fathoms. Abundant. 



Mesodesma Arechavalettoi (Ihering) Pilsbry, n. sp. PI. VI, figs. 15, 16 (about 

 two-thirds natural size). 



Shell shaped much like 31. donacea Lam., but less abruptly trun- 

 cated anteriorly, and wider posteriorly. Epidermis light buff; 

 growth-strias as in donacea. Interior with the pallial sinus very 

 deep, extending beyond the middle of the shell ; lateral teeth weak, 

 the left valve with a well developed /\-shaped cardinal retained in 

 fully adult individuals, with an accessory lamina behind it. Length 

 74, height 40, diam. 23 mm. 



Mar del Plata, Argentina, and Maldonado Bay, Uruguay ; young 

 specimens only from the last named locality. 



This is the shell mentioned as a species of Lutraria in Nautilus, 

 VI, p. 81, It is eaten in Montevideo. 



After deciding the species to be new. I submitted a specimen to 



Professor Wm. H. Dall, who has recently made a special study of 



the Mactracea, and learned from him that the shell has been named 



31. Arechavalettoi by Dr. H. von Ihering. As I have been unable 



to find such a name mentioned in the literature examined in the 

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