572 MEMOIRS OF THE CARNEGIE MUSEUM. 



branchial opt-uinj:;, which lias small jjapilla; on the inner edge. These papillae 

 extend rather far forward, decreasing in size, and disappearing gradually. Palpi 

 moderately large and broad, semicircular, posteriorly with a short truncation 

 forming the posterior margins, which arc not connected. 



Gills rather wide, the inner the wider, chiefly anteriorly, its anterior end 

 immediately behind the palpi; that of the outer gill at the highest point of the 

 attachment-line of the mantle, so that the lower margin of the outer gill is curved, 

 the gill narrowing considerably in front, while the inner is not much narrower in 

 front, and has the lower margin rather straight. Inner lamina of inner gill entirely 

 connected with abdominal sac. Both gills with well developed, strong septa. The 

 non-marsupial outer gill has the septa alternately stronger and thinner, but the 

 alternation is irregular. The marsupium is in the inner gill of the female. Here 

 the septa are more uniform and very strong, but hardly more closely set than in 

 the non-marsupial gill. Where the septa are inserted at the outer limb, they have 

 a swelling on each side, which forms a vertical ridge projecting into the lumen of 

 the water-tubes (See PI. XLVIII, fig. 6). This ridge is less developed in my younger 

 female, yet perfectlj^ distinct. The se.ptsi near the anterior and posterior end of 

 the inner gill do not have marsupial structure, but the marsupial part is very large, 

 occupying nearly the whole gill. 



Genus Monocondyl^a D'Orbigny (1835). 

 D'Orbigny, 1835, p. 37; 1843, p. 611; Simpson, 1900, p. 910; 1914, p^. 1384. 



Normally each valve has only one pseudocardinal tooth, that of the left valve 

 being more anterior. The teeth are more or less depressed (spoon-shaped). Traces 

 of additional teeth may be present, but they are insignificant and variable. 



A key for the species has been given by Simpson (1914, p. 1385), which, how- 

 ever, is artificial and unsatisfactory, and the essential characters apparently have 

 been misunderstood. Moreover, I believe that M. guarayana D'Orbigny (1843, 

 PI. 68, figs. 4-7) does not belong here, but probably to Iheringella. 



The specimens before me all have a cloth-like epidermis, with crowded, ir- 

 regular and anastomosing, concentric lines, which are lamellarly elevated. But 

 very often these fine lamellae are abraded, so that the surface appears smooth. 

 Nevertheless the lamellae are generally preserved in some part of the shell, chiefly 

 on the posterior slope and near the margins, and, even when abraded, they can be 

 easilj' noticed as fine lines. The species at hand, which have been treated by Simp- 

 son have been called "smooth or lightly concentricaUy striate." He distinguishes 

 them according to the outline, orbicular, obovate, eUiptical, subrhomboidal, or 

 subquadrate, but it is extremely hard to draw a line between these terms. 



