284 BULLETIN OF THE 
is such that, if continued, they might join endwise with the inner series, corre- 
sponding to the gills coming from behind the foot, and form a single row. 
The young shell has the teeth of Cetoconcha, the adult loses them entirely, 
having only the usual enlargement of the hinge-line to support the now nearly 
external cartilage, the linear filmy ligament proper outside of the former being 
hardly perceptible, though present. It was on this account, even in the ab- 
sence of an ossicle, that I was led provisionally to describe this as a Lyonsia, 
and perhaps Professor Verrill to call it a Thracia. On further study I found 
in some cases, under the epidermal fibres noted in the original description, 
calcareous granules, especially toward the ends of the shell, while in other 
places there seemed to be no granules. The posterior slope shows more epi- 
dermis than the rest; the outer surface of the shell is faintly iridescent where 
polished, as is the interior. The tendency to rostration at the posterior end 
seems more marked in the older than the younger shells, but differs in differ- 
ent individuals. It is not very marked in any. 
The measurements of the largest specimen in my possession are 13.0 mm. 
long, 10.0 mm. high, and 9.0 mm. in maximum diameter. There seems to 
have been an error in recording or in printing the dimensions of the specimen 
used for the original description. 
Poromya (Cetoconcha) margarita, n.s. 
Plate VIII. Fig. 10. 
Shell small, white, inflated, slightly inequivalve and inequilateral, sub- 
rostrate posteriorly. The right valve a little the smaller ; the lateral outline 
of the valves viewed from within recalls Poromya granulata, but the shell is 
much more inflated ; granulations faint or obsolete except behind, as in the 
last species ; teeth of the hinge obsolete in the adult; anterior and basal mar- 
gins rounded ; behind is a slight concave wave in the margin below, while the 
upper posterior margin descends more rapidly than the anterior one, and is 
subtruncate, and the rostration thus produced gapes slightly; the beaks are 
inflated, but do not rise very high above the hinge-line. Max. lon. 7.3; alt. 
5.5; diam. 6.6 mm. 
Soft parts as in the last species, but the retractile siphon much shorter ; the 
respiratory lamine nine on each side anteriorly, the posterior series about 
eight, and the appendix with about six lamelle. The lower palpi present, 
small; the foot slender, grooved behind, and the other features as in the 
description of the subgenus. 
Habitat. Station 44, off Tortugas, in 539 fms.; Station 221, near Santa 
Lucia, in 423 fms., ooze ; and Station 176, off Dominica, in 391 fms., ooze; 
living at all the stations; the bottom temperature ranged from 39°.5 to 43.°5 F. 
The fragility of the shell is such that nearly all the specimens were broken in 
the trawl. 
