14 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 98 



and subcrescentic. The ligament is rather stout and lies in a deep 

 groove which runs from the umbones halfway to the anterior end. 

 The hinge of the left valve consists of a stout cardinal with a much 

 smaller cardinal posterior to it and partly fused with the dorsal 

 margin ; there is a stout pointed posterior lateral and a smaller ante- 

 rior lateral, both separated from the margin of the shell by more or 

 less deep grooves. The right valve has a short cardinal with a smaller 

 one anterior to it and partly fused to the ligamental nymph ; the 

 laterals are paired, the larger pointed one on the inside, and the 

 smaller one near the margin, the two separated by a deep groove ; the 

 posterior laterals are somewhat stouter than the anterior ones. 



The type, U.S.N.M. no. 472552, measures: Height, 13.7 mm.; 

 length, 14.7 mm. ; diameter, 7.8 mm. It was collected on Clipperton 

 Island, on rocks to the south of the landing place. 



This species differs from the Hawaiian Ctcna bella Conrad and 

 its Pacific analogs by being not as orbicular and by possessing far less 

 conspicuous sculpture, both the radial and concentric sculpture being 

 lower, more obscure, and of equal strength. 



BRACHIDONTES MULTIFORMIS HOUSTONIUS, n. subsp. 

 Plate 4, figs. 4-7 



Shell small, inflated, elongately triangular, the umbones anterior, 

 terminal, the dorsal margin straight, slightly compressed, the ventral 

 side broad and somewhat concave, the posterior portion broad and 

 rounded. The color is deep blackish purple, except for the ventral 

 side and for some distance posterior to the umbones, where the shell 

 is yellowish olive green. The prodissoconch is a smooth, elongate, 

 rectangular purplish shell. Following this the shell is ornamented 

 with many fine radial riblets separated from each other by spaces as 

 wide as the riblets, and crossed by finer concentric lirae, which render 

 the ribs distinctly nodulose in the earlier half of the shell's growth ; 

 in the later half the spiral sculpture becomes more or less obscure 

 and the radial ribs consequently lose their pronounced nodulousness. 

 The interior is a purplish blue, except toward the ventral margin, 

 where it is bluish white. At the anterior end are two purple tubercles, 

 the upper one considerably larger than the lower one. The dorsal 

 margin is strongly denticulate, the denticulations increasing in size 

 toward the posterodorsal angle. The anterior part of the ventral 

 margin, and the posterior margin are crenulated by the ends of the 

 radial ribs. The ligament lies on a long, rather deeply situated shelf. 



The type, U.S.N.M. no. 472858, measures: Height, 5.9 mm.; 

 length, 8.9 mm. ; diameter, 4.9 mm. It, and three other specimens, 



