NATURAL HISTORY. 
81 
GALLERY.-] 
smooth, like Anodon, and the latter has a sharp syphonal inflection. The 
Pleiodon has a series of transverse teeth, like Area , but they are irregu¬ 
lar ; and Hyria and Castalia have compressed crest-like lateral teeth, 
as in Unio. The two latter genera may be distinguished from Unio, 
by having the smaller anterior scar placed over (and not under, as in 
Unio ) the larger scar of the adductor muscle. 
The family of Mycetopodld^ are like the latter, but the shell is 
nearly cylindrical, and widely gaping at each end. The foot is long 
and subcylindrical, expanding into an oblong end, and the two an¬ 
terior muscular scars are widely separated, with the smaller one before 
the larger ; the lobes of the mantle are free, and they have no syphons. 
The family of Trtgoniad^e have a thick cordate shell of a pearly 
laminar texture, with an external cartilage, and the hinge formed of 
two diverging grooved interlocking lateral teeth. The mantle-lobes are 
free, with a large foot, having an acute hooked end, somewhat like the 
cockles, by means of which the animal leaps. 
The family of Arcade are all marine, and have the,mantle -lobes 
free like the former; the foot is compressed, rather variable in shape, 
buTRlways hifid, and furnished with a broad lower edge. The hinge 
of the valves consists of a number of transverse interlocking teeth, 
which appear to be formed by the subdivision of two elongate lateral 
teeth. In general the cartilage is external, arising from diverging 
angular lines marked upon the facet, formed by the gradual thickening 
of the dorsal edges, which causes the umbones to be separated from each 
other as the shell enlarges. The Arete have an angular elongated shell 
with the teeth in a straight line, and a broad short foot, from the end of 
which the animal secretes a quantity of mucus which hardens into la¬ 
mellae, and by means of which it affixes itself to marine bodies. In 
the true Area, Litharca , Senilia, Aroma, and Lunarca , the hinge teeth 
are all equally transverse, the Arete areefongate, subquadrate, and covered 
with a hairy periostraca. The Litharcte are elongate, truncated be¬ 
hind, and live in holes in stones and rocks ; the Senilia have very 
thick shells, covered with a smooth olive periostraca; and Argina are 
ovate, subcordate, convex shells, with a crenated margin, and the front 
group of teeth small and roundish. The Lunarca differ from the former 
in the front group of teeth being replaced by an elevated ridge. In Cu- 
eulltea , Trisis , and Barbatia the teeth on the middle of the line are 
small, of the ends large and oblique The Cuculltea have the scar of 
the adductor muscles placed on a ridge. The Trisis are twisted. 
The Barbatia are elongated shells, covered with a hairy periostraca, 
and the Scaphura are thin shells covered with a thin olive periostraca, 
living in fresh water. The Pectunculi have an orbicular shell, a lunate 
foot, and the teeth in an arched line, their margin is crenated. The 
Lymnopsis have a triangular pit placed on the facet of the valves. The 
Nuculce are free, like the Pectunculi, but the teeth of the hinge form an 
angular line with the cartilages in a triangular pit at its angle ; they are 
pearly within. The Nuculce are ovate wedge-shaped, and the Ledtp. 
elongate and beaked. 
The Pogonopoda attach themselves to rocks and other bodies by a 
bundle of fibres which arise from the front of the base of their foot. 
These fibres are separately formed in a groove in the front of that 
member, and after each has become of a certain consistence, the animal, 
E 3 
