10 PROCEEDINGS OF THE NATIONAL MUSEUM VOL. 125 
subapical notch, and 18-24 spines beneath, unequally divided into 
2 clusters. Anterior ramus of cirrus [ longer than rami of cirri IT and 
III. Comb setae of cirri IT and III lacking basal guards. 
SUPPLEMENTARY DESCRIPTION.—The shell is_ typically conic, 
spreading, commonly eroded externally, and tinted various shades 
of white with deep purple. The radii are extremely narrow or obsolete, 
and often external demarcation of the four parietal plates is virtually 
impossible to recognize. The surface of the body cavity wall is smooth 
or slightly irregular but never ribbed or ridged. 
Externally, the surface of the scutum is eroded, as is the tergum. 
The adductor ridge of the scutum extends nearly the whole length 
of the valve. It is long, low, deeply undercut, and the ridge moderately 
thin. The articular ridge occupies about two-thirds the length of the 
tergal margin, and it is evenly and broadly rounded distally. Prominent 
ridges exist for the insertion of the lateral and rostral depressor 
muscles. Dentition of the occludent margin is limited to 4-6 strong, 
oblique teeth. The number and development of these teeth, however, 
is no doubt contingent upon the age and degree of corrosion of the 
specimen. 
The apex of the tergum is produced into a slight but noticeable 
beak. The articular ridge is high, and its thickness varies. There are 
6-8 strong, well-spaced, inclined crests for the insertion of the depressor 
muscles. 
Parameters of the shell and opercular plates of the five dissected 
specimens (in mm) are as follows: 
shell opercular plates 
carino-rostral 
specimen diameler lateral diameter height seutum height tergum height 
J 19.5 17.2 13.4 OL 6.0 
2 ieyers 1726 122 50.4 6.5 
3 20.6 18.6 12.9 6.1 40 
t 30.8 21.2 16.2 (8 8.6 
5 19 17.6 4.7 2.9 2.8 
On both sides of the shallow, saddle-like groove in the crest of the 
labrum there are four short teeth, although in one specimen dissected 
there were four teeth on one side and only three on the other. Inter- 
spersed among the teeth in the notch and extending laterally along 
the rim of the labrum are short, slender bristles (fig. 2g). Two elongate- 
oval patches of spines extend obliquely from the superolateral margin 
on each side of the crest toward the basicentral portion of the labrum. 
The palps are long, high, and free of setae along their basal margins; 
however, the basal portion of the appendage is uniformly covered 
with ctenoid scales (fig. 2e). The setae on the distal extremity are 
long and setulose whereas those on the superior margin are shorter 
and not covered with setules. 
