24 
NATURAL HISTORY. 
[east. ZOOL. 
( Orthoconchce) which have a symmetrical body, equal valves gaping 
at each end; the mantle partly exposed: they live perpendicularly 
in holes in rocks or sunk in sand, as the Pholadidce, which have no 
cartilage, Gastrochcenidce, Saxicavidce, Pholadomyadce, Solenidce, 
with the valves united by a ligament. 
Table 38. The HeteroconchcB, with a non-symmetrical body en¬ 
closed in unequal valves, with the cartilage in an internal pit, as 
Myadce, Corhiculidce, Pandoridce. 
The second sub-class, or Conchifera Asiphonophora, which have 
the mantle-leaves free, bearded behind or on the whole edge, and 
sometimes with a separate single siphonal opening for the emission of 
the water. 
The Lasiacea, which have the mantle-lobes united with a single anal 
siphon and a large pedal aperture ; SolenomyadcBi Lasiadce, Peradce, 
Tables 39—41. The Z7mowace«, which have the mantle-lobes free, 
only slightly united behind so as to form a single separate siphonal 
opening, close under the hinder adductor muscle. The families 
Lucinidcs, Ungulinidcby which have a cylindrical elongated inferior 
foot, and generally an elongated anterior adductor muscle. The 
families which have a large compressed foot for crawling, as Astartidce, 
Carditidce, Crassatellidce, which are marine. Unionidce, EtheriadcB, 
which are fluviatile and covered with a hard olive periostraca. 
Table 42. The family Mytilidce, which has a small ligulate byssus- 
bearing foot, and a small anterior adductor muscle. 
Tables 43—48. The Fectinacea, with the mantle-leaf free all 
round, without any separate opening for the outgoing current and 
vent. 
Table 43. The Arcacea, with an oblong or roundish shell, equal 
rigid adductor muscles, and the hinge teeth deeply grooved or divided 
into interlocking plates, as the family Trigoniadcdy with a lanceolate, 
subulate, angularly bent foot for leaping. Arcadce^ with a truncated 
foot often dilated at the end, and frequently furnished with a byssus for 
anchoring. 
Tables 44, 45. The Malleacea, with a sub-trigonal shell, with a 
large posterior sub-central adductor muscle, and a byssiferous foot, as 
the families Pinnidce, Pteriadce, Vulsellidce, 
The Ostracea, with orbicular valves, a large central adductor muscle, 
and the mantle bearded round its entire edge. 
Tables 46, 47. The families Spondylidce, PectinidcE, Limadce^ with 
a distinct byssiferous or appendigerous foot, and the lip separate from 
he gills. 
Tables 47, 48. The families Ostreidce, PlicatulidcE, without any 
foot, the body forming a single central mass, and the vent free and 
medial, as in most other Conchifera. 
Table 48. The families Anomiadce^ Placunadce, which have a 
small distinct foot, truncated at the end; the ovaries separated from 
the mass of the body and attached to the inner surface of the right leaf 
of the mantle, the vent on the right side. 
Table 49. The shells of the Mollusca which have no distinctToot 
on the under side of the body, which either live attached to marine 
bodies, float on the surface of the sea, or walk on their head, as the 
