GALLERY. ] 
NATURAL HISTORY. 
23 
specimens of the Gymnobranchous Mollusca , the gills of which are 
exposed or contracted into the cavities in the surface of the mantle of 
these animals. The young animals are furnished with a deciduous 
shell, but the adults are without any. 
Tables 26—30. The shells of the Pneumobranchous Mollusca , 
which respire free air, admitted into a closed chamber, lined with 
pulmonic vessels having a lateral aperture, closed with a valve, gene¬ 
rally placed on the back near the head. The shells are spiral or sub¬ 
spiral, as the families Testacellidoe , Arionidce, Janellidce , Helicidce , 
Veronicellidce, which live on land, and have cylindrical retractile ten¬ 
tacles. 
The families Auriculidce, Lymneadce , which live in the water, and 
have contractile tentacles, and Siphonariadce and Amphibolidce , living 
On the sea-shores within the reach of the tide, but breathing free air 
like the land mollusca. They have the tentacles expanded into a 
broad frontal disk. 
Tables 31—48. The Bivalve shells or Conchifera. The animal 
is enclosed between the tw 7 o leaves of a mantle wdrich encloses all the 
organs with the laminar gills on each side of the compressed body, 
which is generally furnished with a compressed central foot. The 
mouth is in the front of the base of the foot, furnished with two 
laminar lips. The whole is enclosed between tw 7 o shelly valves united 
together by a ligament on the hinder part of the dorsal edge. 
Tables 31—38, contain Conchifera , Siphonophora , which have the 
mantle closed behind, and furnished with two apertures, the lower for 
the admission, and the upper for the emission, of the w 7 ater from the 
mantle cavity. 
Tables 31—36. The Veneracea , which have two more or less 
elongated siphonal apertures, and the gills not produced into the lower 
or inhalent siphons. 
Tables 31—36, contain the shells ot the families wdiich have a com¬ 
pressed foot for crawling, as Veneridce , Cyprinidce , GlauconomidcF., 
Corbiculadce, Cyrenellidae, Petricolidce, Tellinidce, Paphiadce, Glot- 
tidce, Anatinidce , Mysiadce, Mactradae , Anatinellidce , Chamadce, 
Muteladce. 
Cardiadce , which have an elongated acute conical foot angularly- 
bent in the middle fitted for leaping; the families w 7 hich have a truncated 
foot dilated at the end for anchoring or crawling, as Ledadce, Modio - 
aridce ; the Preissenidce, and Goleommidce, which have an elongated 
slender strap-like foot, spinning a beard or bvssus for anchoring the 
animal; and lastly, the family Tridacnadce, which have a very small 
rudimentary byssiferous foot. They are also peculiar among all bivalve 
shells for having the very large hinder adductor muscle placed far 
forward near the centre of the lower edge of the shell. The pedal 
opening is small in front near the umbo, and the anal siphon is behind 
and above the adductor muscle as in other bivalves, though it appears 
in front, being displaced by the position of the adductor muscle. 
Tables 37, 38. The Pholadacea, or Conchifera, with the mantle 
closed in front, more or less elongated, united siphons. The gills 
large, produced into the low 7 er or inhalent siphon. Those families. 
