52 CLASS III. GASTEROPODA. ORDER IV. PLEUROBRANCHIATA. 
‘‘Parce que le seul individu que nous avions vu, avoit sa coquille, proba- 
blement par artifice, collée ou attachée sous le pied.” 
The shell of Umbrella may be described as being external, dorsal, 
orbicular, and smooth ; the upper surface is rather convex, and generally 
white, the vertex being very small, and situated near the centre; the 
under surface is rather concave, and has the central disc of a brown- 
yellowish colour, with an irregular, continuous, muscular impression 
running around it. 
Example. 
Pl. CLV. Fig. 1 and 2. 
Umsretta Inpica, Lamarck, Anim. sans vert., new edit., vol. vii. p. 573. 
Chemnitz, Conch., vol. x. pl. 169. f. 1645 and 1646. 
Patella wmbellata, Gmelin. 
Family 3. APLYSIANA. 
Testa vel cornea, vel calcarea, intra pallium, branchiarum tutamine, ce- 
lata. 
The Aplysiana constitute another small family of mollusks in which 
we find no little difficulty in establishing a good generic division ; for 
whilst De Férussac, Rang, and Deshayes have advocated the propriety of 
including them in one and the same genus, Lamarck, Sowerby, and De 
Blainville have continued to separate them. They resemble the Semi- 
phylldiana in their general anatomy, and like most of that family, have 
the shell concealed within the fleshy substance of the mantle ; instead, 
however, of being inserted on the back, it covers the branchial cavity. 
The branchiz agree in being situated on the right side only of the body, 
but not in their arrangement: in the Semiphyllidiana they are placed 
under the edge of the mantle; but in this family they are situated in a 
particular cavity, which the shell is destined to protect. 
