366 PLEUROBRANCHIDA. 
Famity PLEUROBRANCHID. 
Shell lim pet- like or concealed, rarely wanting ; mantle or 
shell covering the back of the animal ; gill lateral, between the 
mantle-margin and foot; food vegetable, stomach extremely 
complicated. 
The animals of this family have no upper jaw, the lingual 
membrane is armed with numerous short teeth, arranged in a 
quincunx; there are four stomachs, the second of which is fleshy, 
and sometimes furnished with bony pieces, and the third is pro- 
vided internally with prominent longitudinal lamellae; the 
intestinal canal is short. The species are tolerably numerous, 
occasionally of large size and adorned with varied colors; they 
are mostly inhabitants of the high seas. 
P}LEUROBRANCHUS, Cuvier. 
Etym.— Pleura, side, branchia, gill. 
Syn.—Berthella, Bl. Oscanius, Gray. Haliotinella, Souverb. 
Distr.—22 sp. South America, Norway, Britain, Mediterra- 
nean, Red Sea, Ceylon. P. citrinus, Ruippell (Ixxxix, 82, 83).. 
Shell internal, large, oblong, flexible, slightly convex, lamellar, 
with a posterior, subspiral nucleus, 
Animal oblong, convex ; mantle covering the back and sides, 
papillated, containing spicula; foot large, separated from the 
mantle by a groove; gill single, free at the end, placed on the 
right side between the mantle and foot; orifices near the base 
of the gill; head with two grooved tentacles, eyes at their outer 
bases; mouth armed with horny jaws and covered by a broad 
veil with tentacular lobes. : 
PLEUROBRANCHAA, Meckel.  (Pleurobranchidium,  Blainv.) 
Mantle-margin very narrow, not. concealing ‘the vill; dorsal 
tentacles ear-like, oral veil tentaculiform. Py Meckelii, Blainy. 
Cixi 0): 
KooNsIA, Verrill, 1882. (Dedicated to B. F. Koons, U. S. Fish 
Commission.) Allied to Pleurobrancheea, 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 reproductive 
organs appear less complicated than in Pleurobranchwa. The 
verge is armed with small hooks, but the spicule, present in the 
latter genus, is not protruded, if present, in the specimens of 
Koonsia; urinal opening at the anterior root of the gill; between 
this and the verge, some specimens show a small. opening and a 
low ‘papilla, but none show the large opening and Tong flat papilla 
