ORDER: TECTIBRANCHIATA. 45 
shell is calcareous. ‘They are found in the Mediterranean and 
in the Indian Ocean. (Dolabella Rumphii, Cuv. Ann. du 
Mus., &c.) 
NoTARCHUS, Cuv. 
The lateral crests united and covering the back, a longi- 
tudinal emargination excepted, that leads to the branchiz, 
which have no mantle to cover them, but are otherwise like 
those of the aplysiz; the rest of their organization is always 
the same. (Notarchus gelatinosus, Cuv., &c.) 
BURSATELLA, Blainv. 
The lateral crests are united in front in such a manner as 
only to leave an oval aperture for the transmission of water to 
the gills, which are also deprived of a protecting mantle. 
(Bursatella Leachii, De Blainv.) 
These two genera, however, probably form but one, 
AKERA, Miiller. 
The branchiz covered as in the preceding genera, but their 
tentacula are so shortened, widened, and separated, that they 
seem to be totally wanting, or rather to form a large fleshy and 
nearly rectangular shield, under which are the eyes. In 
other respects, the hermaphroditism of these animals, the 
position of their genital organs, the complication and armature 
of their stomach, and the purple liquid effused by several of 
their species, approximate them to the aplysia. The shell, of 
such as have any, is more or less convoluted, but with little 
obliquity, and without a projecting spire, emargination, or 
canal; the columella, making a convex projection, gives a 
crescented figure to the aperture, the part opposite to the 
spire being always the broadest and most rounded. 
M. de Lamarck names those in which the shell is concealed 
in the thickness of the mantle BuLLAA. It has but 
