368 NUDIBRANCHIATA. 
Distr.—6 sp. Canaries, Mediterranean, India, China, Sand- 
wich Islands. Fossil, 4. sp.. Jurassic—; United States, Sicily, 
Asia. U. Mediterranea, Lam. (lxxxix, 87). U. Indica, Lam. 
(Ixxxix, 88). 
Shell limpet-like, orbicular, depressed, marked by concentric 
lines of growth; apex subcentral, oblique, scarcely raised ; 
margins acute; inner surface with a central colored and striated 
disk, surrounded by a continuous irregular muscular impression, 
It has a minute sinistral nucleus. 
TytopinA, Rafinesque. 
Distr.—3 sp. Mediterranean, Norway. Fossil,1 sp. Tertiary. 
T. punctulata, Raf. (Ixxxix, 89). 
Shell limpet-like, depressed, apex subcentral, with a minute 
spiral nucleus. 
Animal oblong, foot truncated in front, rather pointed behind ; 
dorsal tentacles ear-like, with eyes sessile at their inner bases ; 
oral tentacles broad; branchial plume. projecting posteriorly 
on the right side, 
OrpER NUDIBRANCHIATA. 
Animal destitute of a shell except in the embryo state ; 
branchie always external, on the back or sides of the body. 
Sexes united. 
The Nudibranchiate sea-slugs are found on all coasts where 
the bottom is firm or rocky, from between tide-marks to a depth 
of fifty fathoms ; a few species are pelagic, crawling on the stems 
and fronds of floating sea-weed. They have been found in the 
Icy Sea, at Sitka, and in the sea of Ochotsk; in the tropical and 
southern seas they are abundant. They require to be watched 
and drawn whilst living and active, since after immersion in 
spirits they lose both their form and color. In some the back 
is covered with a mantle (viii, 39), which contains calcareous 
spicula of various forms, sometimes so abundant as to form a 
hard shield-like crust. The dorsal tentacles and gills pass 
through holes in the mantle somewhat like the “ key-hole” in 
Fissurella. In others there is no trace of a mantle whatever. 
The eyes appear as minute black dots, immersed in the skin, 
behind the tentacles; they are well organized and conspicuous’ 
in the young, but often invisible in the adult. The dorsal ten- 
tacles are laminated, like the antenne of many insects ; they are 
never used as organs of touch, and are supplied with nerves. . 
from the olfactory ganglia. The Doridide are distinguished by — 
having a short and wide lingual membrane with numerous similar — 
teeth; the Molids haye a narrow ribbon with a single series of 
larger teeth. In Dendronotus a large central tootha is Hone: city 
a few sma!] denticulated teeth. setarinG -" qe" eet 
, 
