A. E. Yerrill — Catalogue of Marine Mollusca. 
545 
it is broadly flaring and convex, with a slight rounded angle about 
at the middle ; anteriorly it is a little narrower and evenly rounded ; 
the columella margin is slightly excurved, with a thin edge, in front 
of the middle, and is reflected against the body-whorl, where it joins 
it, leaving a slight groove behind it, and winding into the shell, it 
forms a distinct, raised spiral fold, separated from the more promi- 
nent, inner surface of the body-whorl by a concave groove. 
Length, 10 - 75 mra ; breadth, in middle, 8 mn ‘ ; breadth of aperture, 
Station 921, in 65 fathoms, two living specimens. 
Koonsia, gen. nov. 
Allied to Pleurobranchcea , with which it agrees in the character 
of the head, tentacles, proboscis, and gill. It differs in having the 
back swollen and overhanging both on the sides and posteriorly, and 
a distinct mantle-edge all around, with a wide groove between it and 
the foot posteriorly, as well as laterally ; the foot is narrower and 
prolonged posteriorly, with a specialized glandular area, near the end, 
beneath, and a conical papilla above, near the tip. The external re- 
productive organs appear less complicated than in Pleurobranchcea. 
The verge is armed with small hooks, but the spicule, present in 
the latter genus, is not protruded in any of our specimens of Koon- 
sia, if present ; the urinal opening is at the anterior root of the 
gill; between this and the verge, some specimens show a small open- 
ing, and a low papilla, but none show the large opening and long- 
flat papilla, present in Pleurobranchcea , and usually well displayed 
in alcoholic specimens. Anal opening behind the base of the gill. 
Gill large, bipinnate, fully exposed on the right side, between the 
mantle and the foot. 
This genus is dedicated to Mr. B. F. Koons, of the U. S. Fish 
Commission, in 1880 and 1881. 
Koonsia obesa Verrill, sp. nov. 
Body large, stout, broad, with a large, swollen back, smooth and 
white in the preserved specimens, and defined by the mantle-edge, 
which forms a rim along the lateral and posterior borders. Head 
large and broad, with two short, flat, posteriorly grooved, anterior 
tentacles, one at each corner ; the anterior mantle-border runs between 
them, and supports a row of small papilla;. Posterior tentacles 
short, stout, flattened, ear-like, with the outer edges incurved, form- 
